Forensic Science includes those aspects of scientific fields which are applicable and useful for the purpose of law. A Forensic Science Laboratory pertakes the relevant jobs.

Criminal investigation, in which a medical man participates, are to a great extent dependant on the services of a Forensic Science Laboratory. Hence a medical man, particularly, he who is expected to give medicolegal service must have an idea about the scope and extent of the nature of services rendered by these laboratories.

FORENSIC SCIENCE LABORATORYIn our country, there are varieties of these laboratories, scattered all around. Some of these laboratories are under the control of Union Government agencies; some under the different State Governments. Centrally controlled laboratories take up both specialized and specific type of investigations or investigations generally required in most criminal cases. Almost each state has a State Forensic Science Laboratory. Depending on the workload, State Forensic Science Laboratories may have branches to cover distant comers of the states.

The State Forensic Science Laboratories are overburdened with work-load and cannot do justice to the different parts of a state, when, only one laboratory caters to the whole state. It is necessary that each district of a State be provided with a laboratory so that there will not be undue delay in the process of investigation.

Set up of a Forensic Science Laboratory —

For wide range of service, a Forensic Science Laboratory should have the following working sections:

1. Toxicology Section — This section undertakes the examination of different materials including different body organs or viscera and fluid or other exhibit for detection of various poisons or drugs. Sometimes separate establishments are provided to share the load of this section of the laboratory.

2. Serological Section — This section undertakes blood group tests for different suspicious stains, body fluid and blood.

3. Biological Section — Identification of different body fluids as to their nature, examination of hair of different origin, plant materials, are done in this section.

4. Molecular Biology Section — With the use of D.N.A. test in forensic investigation, many F.S.L. have started this comparatively new unit.

5. Physical Investigation Section — Various physical evidences collected from a sence of crime or elsewhere, are tested in this section to know as to the nature of the physical evidence.

6. Ballistic and Explosive Section — All about the firearms, missiles, gunpowder and other explosive materials are examined in this section.

7. Finger Print Section —Development and study of latent and chance finger prints are the job of this unit. At many places this section is independent of FSL Services.

8. Polygraphy Section — In many countries, service of polygraph or lie detector is incorporated in Forensic Science Laboratory.

9. Document Analysis Section — Handwriting, typewriting and examination of alleged forged documents, currencies are undertaken in this section of the laboratory.

10. Photography Section —Many of the evidences detected from different exhibits or materials require to be photographed. Hence a photography section is imperative for such a comprehensive laboratory.

11. Lastly, each Forensic Science Laboratory should have a mobile evidence collection unit with provision of vehicle, preservative and other required materials.

Of the services rendered by different sections of Forensic Science Laboratory, a medicolegist is concerned mostly with services of Toxicology, Serology, Biology, Molecular biology, Ballistic, Explosive and Polygraph section. Hence, it is desirable that, he should have some idea about the technique and equipment used in these sections. The following paragraphs are aimed at this end.

For detection of poisons, drugs and different chemicals, apart from usual chemical and biochemical methods, the following analytical techniques are applied, which are of course much more dependable and satisfactory:
1. Chromatography
2. Electrophoresis
3. Spectrophotometry.


Chemical substances, may it be gas, liquid or solid, spreads around a media or in a suitable environment. This spreading character is different for different materials in different media. This property of chemicals is suitably used by different chromatographic tests, for qualitative and quantitative assessment of different poisonous drugs and chemicals.

Different Chromatographic Techniques:

1. Column Chromatography : In this, a glass cylindrical apparatus is packed with different absorbents, at different levels. The material to be tested is placed at the top of the absorbents, inside the glass cylinder. Then a suitable solvent is allowed to pass very slowly from the top. The materials to be used as absorbents are silica gel, aluminium, charcoal etc. The solvent chosen, depends on the suspected nature of the material to be tested. On being dissolved in the solvent, the test material will pass down and depending on its nature, will be absorbed at some level of the absorbents. This process may directly help to detect the tested material or may help to collect the material in pure form after evaporation of the solvent, from the level of the column which has absorbed the material. In the later event, more specific tests can be taken up to know the material.

2. Paper Chromatography : Special strips of absorbent paper (porus, filter paper) is used for the purpose. The testing material is spotted at one end of the strip of the paper. By its side, a pure sample of the suspected testing material is used as control. This end of the paper is dipped in a solvent in a glass tank. The solvent is soaked by the paper and rises in its height. On way, it dissolves the testing material and its pure control and carries part of them, up in the strip along with it. The ratio of the length of the rise of the testing material to the length of the rise of the solvent in percentage is taken into consideration and the value is expressed in Rf, which is compared with the control as well as with a standard chart. To identify the material, for the invisible stains of the test material on the paper, suitable colouring spray is used.

3. Thin Layer Chromatography : The principle is to a great extent similar to paper chromatography. Instead of specific paper strips, thin layer of absorbent materials like silica gel or aluminium gel preparation is used on glass plate. The sample to be tested and a control sample of pure form of the suspected material in the sample, both are placed side by side at one end of the chromatographic slide on the absorbent surface and this end is dipped into a suitable solvent in a glass container. But the solvent should not touch the testing material and the control. The solvent is allowed to rise upto 10-15 cm. which occurs between some minutes to 2 hours depending on the solvent. Then the plate (The chromatogram) is taken out and dried and exposed to U.V. rays which identifies some materials. After that, the absorbent is separated out, and the material is also separated by using some suitable solvent material, which is subjected to further tests like spectroscopic tests for proper identification of the material. Simple spraying of colour-producing reagent may help by making visible the areas of extension of the test material, with their control for comparative study.

There are certain advantages of T.L.C. (Thin layer chromatography) over paper chromatography. T.L.C. can identify very small quantity of the material in a very short period. The material from the chromatograph can be easily separated and subjected to other tests. Most of the chemicals or poisons can be identified by this method.

4. Gas Chromatography : In gas chromatography the material to be tested is mixed in its gaseous or vapour form, with some inert gas like nitrogen or helium and then passed through layers of different absorbing particles arranged in a column (virtually an appliance used in column chromatography) which absorb different chemicals present in the testing material and then allow the different components of the gas or vapour to come in phases out of the column, which are then passed through a detector e.g., an electron captor detector or a flame ionization detector. The electrical signal is fed into a recorder, after amplification of the signal in flame ionization detector. Flame is used to ionize the components of the gas for generating electric signal. The signal is recorded on a strip of chart recorder. The vertical ordinate of the graphic chart records the power or electric strength or electric response from the ionized component and the horizontal axis records the time of each such response and thus, that gives the retention time of the components inside the column. The time required for rejection of the components help to identify the material.

Gas chromatography is most suitable for testing gaseous substances. If in liquid or solid form, the substance is to be converted to its gaseous form first and then subjected to the test. This test also gives quantitative assessment of the material, which is most accurate. The peak of height (vertical ordinate) recorded, represents quantity.

In pyrolysis gas chromatography, solid substances are subjected to very high temperature, so that, the material is converted to complex gas which is then fed into the gas chromatography column along with an inert gas. This type of chromatogram is termed pyrogram. The graphic record of a chromatogram or a pyrogram acts as a “finger print” for the substance, indicating that in this way there cannot be any wrong in knowing the nature and quantity of the matter.


1. By this method different protein components of different body fluids are identified. The protienous components of the testing material pass through a stationary phase on application of electric charge. The charged components move a certain distance. The separated proteins in the stationary phase are stained with colouring agents which show characteristic bands, each of which is individualistic for a protein. The stationary phase is made of starch or agar gel on a glass slide. The testing material is placed at one end of the stationary phase (gel layer on the slide). Electrodes are attached at both ends of the gel slide. Electrically charged protein component moves on the phase plate and at the end, the gel plate is treated with the colouring agent for the protein which causes appearance of visible characteristic bands depending on the characters of the protein.

Starch or agar gel electrophoresis is used for serological tests for identification of human serum or for specific blood group antibody, by placing antigen and antibody on separate holes at short distance and causing electrophoresis movement of the antibody across the gel phase, to reach the antigen side and vice versa. The precipitation and agglutination reaction can clearly be studied in the gel media.

2. In paper electrophoresis, whatman no. 3 paper is used instead of gel plate. The sample to be tested is applied at one end of the paper strip in the midline. Buffer solution is applied so as to make the whole paper strip wet, except 1 cm. breadth area on each side of the spot, made with the testing material. The end of the strip nearing the spot is now dipped in the same buffer solution, in a vertical electrophoresis tank. Electrophoresis is continued for about 15 hours. The spot made with testing material and its surrounding which was kept dried, soon gets wet by the buffer solution, equally from all sides. As the electrophoresis is continued, there is displacement of the protein which makes specific band at specific distance, which is characteristic for the protein.


Every substance have the capacity to absorb light rays of various wavelengths, both in the visible as well as in the invisible range. In the visible wave length range, absorption of some components of rays and reflection of others gives the specific colour to a substance. On being examined by a spectroscope, dark absorption bands are seen, representing the absorbed splitted rays of specific wavelengths. Light rays are splitted according to their wavelengths, by a prism and passed through the substance to be examined. The absorption of specific wavelength by the substance causing appearance of dark bands in specific zones of the visible light rays range, helps identification of the matter without any chance of mistake. In the practice of Forensic Medicine, use of ordinary spectroscope is helpful to identify haemoglobin in its different forms of chemical state and combination.

Colorimeter is a type of spectroscope, in which by using filters, some of the light rays of specific wavelength range are allowed to pass through the testing matter and others are restricted. When the rays of specific wavelength pass through the testing substance, absorption of some of these rays of specific wavelengths by the substance makes it easy for photo-electric detector to detect the range of wavelengths of rays absorbed. The source of light radiation in simple spectroscope and ordinary colorimeter is tungsten bulb.

Ultraviolet and Infra-red spectroscopy

As in case of visible light rays, the invisible light rays e.g. those with more frequency and less wave length like U.V. rays, X-rays, gamma rays and those with less frequency and more wavelength like infrared rays, microwave rays and Radiowaves are also absorbed by different substances which cannot be visualized by naked eye or by ordinary spectroscopic examination. They can be appreciated and in their case range of absorption can be measured by more sophisticated spectrophotometers. Such sophisticated spectrophotometers have components like, monochromators, sensitive electronic detectors with amplifier and a recorder, to measure the amount of electric energy liberated (and amplified) from photons which are emitted by the testing substance, when placed in a suitable media, through which the monochromatic rays pass.

For ultra violet spectra photometer, the light source is either a hydrogen or deuterium discharge lamp, and for infrared spectrophotometer a heated moulded rod of special type is more suitable.

In the monochromator, a prism splits the light rays according to their wavelengths and slit allows to pass rays of suitable wavelength range.

The testing matter is dissolved in a solvent and glass cells or quartz cells are used in the solution for absorption of visible and U.V. rays respectively. For Infrared rays, cells made of sodium chloride and potassium bromide are suitable.

Photons liberated from the surface of the cells in the solution strike the photoelectric detector, get converted to electrical signal and then are amplified. The signal is represented graphically and thus the electric signal is then recorded graphically by the recorder. This graph when compared with a graph of U.V. or I.R. rays, gives the nature of absorption of the wavelength energy by the test material which coated the cells. Thus, the matter is identified from the graph obtained.

The graph obtained in U.V. spectrophotometry is easy to study but not conclusive about the nature of the material tested because many substances may exhibit similar energy and thus the recorded graph from them also may be similar. Confirmation about the materials may, after this test be done by T.L.C. which becomes easier, once some idea about the testing material is available by U.V spectrophotometry.

In case of Infra red spectra photometry, the matter is a bit different. Here the absorption bands are numerous, which gives a very complex pattern of the graph for each material tested. Hence the spectrum represented in the graph, gives so many characteristic complex patterns that, the graphic prints can be safely accepted as confirmatory for some substance which, in pure form, is also subjected to the same test as a control.

Drugs and other trace evidences can be detected by spectrophotometry.

Mass Spectometry:

Mass Spectometry is more sophisticated a test which detects a substance in minute quantity. The testing material in trace amount is placed inside a high-vacuum chamber and is then collided by high energy electrons. This causes loss of electron from the molecules of the testing material which get positively charged. These molecules break down instantaneously into numerous fragments, which are then separated according to their mass, by passing them through an electromagnetic field. The graphic presentation of the minute particles of the mass as obtained subsequently, by the same process as applied in case of U.V. or I.R. spectrometry gives a complex pattern with qualitative and quantitative assessment of the material tested. The lines in the graph represent the mass and the height of the lines represent the quantity of each fragmented mass. Thus, the whole graph acts as an unique picture for a material. Mass spectroscopy is applicable only in case of testing some pure material. In case of possible admixture or impurity, the components of the material may first be subjected to fractional separation by gas chromatography and then tested by mass spectrometry for identification and estimation of each type of matter present in the complex. Chemical ionisation mass spectrometry is an easier process to identify components of a mixture without application of gas chromatography beforehand.

Emission Spectrograph:

Every element, on being excited appropriately, emits light spectrum. The spectrum can be separated and recorded by photography.

The element to be tested is placed in between two carbon electrodes to vaporise and excite the particles of vapour which then emits light which is then passed through a lens and then through a prism to spread the spectrum which is again passed through another lens to focus it on a screen to be photographed for further study and preserving as record. None of the elements give similar line of spectrum and from the bands of spectrum and dark bands the element can be identified.

In case of a compound, many lines of spectra are recorded having a specific type for each element. From the combination of different lines of spectra, the elementary combination of the component can be assessed and then the compound can be identified and compared with another by similar test. A modification of the process may be adapted by use of laser beam at the initial phase to vaporise the compound. Rest of the test i.e., exciting of the elements of the compound by carbon electrodes and the subsequent process is same as in the other variety of emission spectroscope.

Atomic Absorption Spectrophotometry:

Here, an element or compound is subjected to the contact of Acetylene air flame or a graphite furnace or a heated strip of metal for vaporisation of atoms of the specimen. Radiation from a light source of similar type is directed towards the vaporised atoms which absorbs the radiation energy with displacement of electrons from the outer circle of the atom. This movement of the electrons emit energy which is passed through a manometer and then the selective radiation is processed through the detector and recorded graphically, which speaks for the element’s presence in the component.


When an atom is bombarded with a neutron, isotope of the atom with increased number of neutron develops. However, most of such newly formed isotopes are decomposed and there is radioactive decay of the isotopes. There is radioactivity in the process with liberation of alpha, beta and gamma rays. Alpha rays are positively charged particles, beta rays are electrons and gamma rays are electromagnetic radiation. In a neutron reactor, neutron is bombarded to an atom which enters the nucleus of the atom and during disintegration the gamma ray radioactivity is liberated. This has characteristic energy value which is assessed to know the atom or the element.

Many elements can be studied at a time inside a reactor. The characteristic energy value of gamma rays tells about the type of the element and the intensity of gamma ray radiation tells about the concentration of the element in a substance. The measurement of the gamma rays energy involves a sophisticated testing technique which needs a 1. detector for the rays, 2. a multichannel analyser to know the type of energy of the radioactivity and also 3. a recorder.

Many elements (or many complex organic substances can thus be studied at a time. It identifies an element or compound in 1/1000000000th (one billionth) of a gram. But the apparatus is a too costly one and the analyser has to be very efficient and experienced.

Neutron activation analysis is useful to detect trace elements in hair, gun powder deposited around a firearm wound, primer contaminating the hand of user of a gun etc.


X’ray defraction from atoms of a substance is photographed in X-ray plates, which gives specific patterns for each individual material. The material should be available in crystalline form to be detected by X-ray defraction.

Stereoscopic Microscope:

In stereoscopic microscope two eyepieces give pictures of two planes of the material examined, giving a 3 dimensional image. It is a reflection microscope which gives surface view of trace elements present on the surface of some other object. The field of view being, wider a wider surface can be examined. The magnification is 15 to 125.

Comparative Microscope:
Fluorescent Microscope:

By this type of microscope, only certain specific items can be studied. To have a more general use, the specimen to be examined, is to be treated with substances which fluoresce, so as to be visible by this microscope. The specimen treated with fluorescent agent, after absorption of invisible short wavelength, high frequency rays (e.g. U.V. rays), cause emission of rays of longer wavelength and lesser frequency, so as to make the emitted rays visible due to presence of fluorescent agent.

Polarising Microscope:

In polarising microscope, polariser and analyser are used, the former in between the source of light and the object of study and the latter (the analyzer) in between the object and the eye piece.

Though light passes in straight lines in all directions, some materials allow the light (passed through it), to travel in one plane only and not in all directions. This is the function of both polariser and analyser. A polarising lens turns the path of light rays passed through it, to pass in one direction only. The analyser (which is actually another polariser), is used to ascertain, whether the polariser limited the path of light passed through it, in one plane or not. When the analyser is placed in a perpendicular plane, in comparison to the placement of the polariser, then it can be understood if light from the matter tested, was polarised by the polariser or not, as in such a case the analyser being placed in the stated manner, will change the pathway of the light rays coming in one plane, after passing through the polariser and will not be visible. Placement of the analyser then, in the same plane of the polariser will cause the light rays to pass in the same direction and the matter will now be visible, if looked through the microscope. Thus, by this method, nature of dispersion of light rays by the tested specimen will be known. In this way a polarising microscope tells about the nature of the specimens tested (mostly minerals), from their nature of dispersion. An ordinary microscope cannot be used for this purpose.

Electron Microscope or Scanning Electron Microscope

This is an example of great advance in the study of surface of some objects, so as to identify it, or any trace material, upto the level of elements. The use of S.E.M. (Scanning Electron Microscope) gives highly magnified detail picture of the surface of the material tested for trace element, as well as, a graphic presentation of electrons emitted by the element present in the testing substance which gives clear identification of the trace elements.

Beam of electrons is emitted from a hot tungsten filament. These electron beams are focussed on the surface of the testing material by means of electromagnets. The focussed primary electrons cause emission of electrons from the surface of the testing material. These emitted electrons are scanned, amplified and fed into a cathode ray tube to be converted and focussed on a screen to produce magnified pictures, as it happens during picture formation on a T.V. Screen.

The magnification of the image may be upto 100000 times than the real dimension of the testing particles. The depth of the picture is also quite high, about 300 times more than other surface microscopes and the magnified image of surface of the testing material has a 3 dimensional stereoscopic appearance.

This helps identification of minute trace elements present on the surface of a matter, or in other words, gives an enormously magnified surface of some minute invisible trace element, the exact nature of which is required to be known in Forensic practice.

Electrons, striking surface of the testing material, also produce X-ray which is deflected from the surface. With the help of an X-ray analyser and recorder, the amount of X-ray emitted, and its character can be known which will tell about the elements present on the surface of the examined sample. This way, the metallic trace from the primer or gun powder contaminating the hands of a shooter can be detected, as to whether the material contained lead, antimony etc.

In fact electron microscope can be utilized for detection of extensive range of trace elements available in the dress and other articles of the accused, victim and at the scene of crime.


Polygraphy is the process which is used in medical practice for comprehensive study of functioning of different body systems wth particular reference to, circulation, respiration and peripheral nervous response. This technology has been attempted in forensic investigation process. The basis of its application is the fact that with mental excitation or stimulation there is alteration of these body functions due to autonomic, particularly sympathetic excitation.

Basing on this principle, polygraph, which indicates the functioning levels of the above noted systems, has been used to know whether a suspect or an accused of a case is deceptive while facing interrogations during the investigation, so that subsequent investigation process can be channelled through right way. For this purpose, the persons to be so examined with the help of a polygraph should be so done in his complete physical and mental relaxation stage, without any factor acting on him to influence the responses, except which should naturally occur while giving a deceiving or false reply.

The person is made to sit on a chair and the accessories of the instrument are properly attached on different parts of the body. An arm cuff is placed around the arm for recording blood pressure and pulse rate and pulse features. An elastic belt is placed around the chest to measure the rate and amplitude of respiration with deviations and an electrode connection is placed, one on the tip of one side index finger for recording galvanic skin reaction (Galvanic current is used for the purpose). The response is recorded graphically on a single paper from where different adverse responses, the intensity of responses, and the time and extent of exciting reaction, can be studied.

The subject to be examined is to be prepared without any premedication. The preparation is more a mental preparation than otherwise. Certain subjects are naturally unsuitable for this test, for example, subjects with psychotic personality, over reactive personality, drug addicts, persons suffering from gross abnormality of any of these three systems to be tested and persons who are by nature deceptive, restless and noncooperative. These subjects require special preparation and need time to be fit for the test. They are not suitable for ready examination.

Preparation of the subject (who are suitable for ready examination). The person is subjected to pre-examination interview during which its purpose, aim, the process of polygraph examination to be followed, should be explained to him to his optimum understanding. For satisfactory result of the test, the tester should have the knowledge of the incident. The subject should be informed that, he would be asked certain questions, and he is to answer the questions as ‘yes’ or ‘no’. For this the questions will be of suggestive in nature. The subject has nothing to be apprehensive about any wrong study and result of the polygraph. If he deceives then, that will be reflected in the test. In the second stage he should be made acquainted with the questions and he has to understand the questions well so as to give ‘yes’ or ‘no’ answers. Ideally, not more than 10 questions should be asked to him in the same sitting. Initially three categories of questions are asked.

1. Irrelevant questions : These are the questions which have no bearing with the incidence of offence in any way. For example – Is your name Mr. ‘X’ ? Are you 25 yrs. in age ? Are you an usual inhabitant of ‘Y’ area ? Do you work in ‘Z’ firm ?

2. Relevant questions : Relevant questions are mostly directly implicating him with commission of the offence or suggestive of having knowledge about some aspects of the offence. For example, ‘on 15-8-93 at 4-30 p.m, you stabbed Mr. A at his residence or saw Mr. B stabbing Mr. A, on 15-8-93 at 4-30 p.m’. ‘You have robbed N.D. bank of Rs. 4 lacs’. The answers for the relevant questions should be “yes” or “no”.

3. Control questions : For proper understanding and interpretation of the graphic curves imprinted with answers and mental reaction of the subject to different relevant questions, control question are asked, which are mostly generalised in nature, related to some minor bad acts which the person might have committed some time in his early life and should have not forgotten. E.g., ‘Have you at any time during your childhood, stolen some money from your mother’s purse for purchasing some playing materials’ ? ‘Did you ever think to make money by way of bank robbery or some such way’ ?

In case of deceptive answers, adverse reactions are reflected in different ways in the graphic presentation of the body functions. Irrelevant questions are asked to facilitate comparison between the reaction to a correct answer and that to a deceptive answer. Interpretation of reaction to answers to controlled questions help further to assess whether the person is generally reactive to any of questions which he feels may indirectly implicate him to the present offence in question, which has not been committed by him. Thus, a negative answer for both relevant as well as control questions with similarity in abnormal response will rather be taken as the person’s adverse nervous reaction to any incriminating question or affair.

The questions should be so arranged that the graphic response can be easily compared and becomes conspicuous. Thus, an irrelevant question should be followed by a relevant question, to be followed by an irrelevant question again and then should be followed by a control question. All these should be so done to allow the person to come back to normal receptive stage, after giving a deceptive answer to a relevant question, so as to make him ready for another relevant question. The purpose of the control questions have been narrated above.

Examination proper : The person to be examined is made to sit on a polygraph chair in such a way that he faces the instrument and the operator faces him so that during questions and answers he can observe the facial reactions of the subject.

The different settings of test constitutes :- (i) first test (ii) card test (iii) 3rd test (iv) mixed question test (v) yes test (vi) guilt complex test (vii) repeat test (re-examination test) (viii) peak of tension test.

1. The first test is carried on after the subject settles after the appliances and instruments are attached to his different body parts and after he is explained about the type of questions and the type of reactions which will be obtained, if he lies.

2. Card test : In case interpretation becomes difficult on the findings of the first test, the card test may be performed to know whether he has lied during the first test.

The person is subjected to this test when the findings of the first remains ambiguous or if no conclusion can be drawn from the findings. Seven playing cards of different numbers are used for this test. The person is asked to pick up one out of them without looking at the number. Then he will see the number of the card he has picked up. The card is taken back from him and mixed with the other six cards. Then all the seven cards are handed over to him with instruction that, he should take up each one individually and when asked the question. “Is that the card you picked up earlier ?” he should answer “No” in all cases, including the question relating to the card he actually picked up earlier.

Thus, one of the “No” answers must be wrong, and graphic change due to such wrong answer becomes helpful to make a comparative study with graphic pattern related to his answers for relevant questions asked during the first test. If the responses are similar, then, it become strongly suggestive that, he lied during the relevant question during the first test.

3. A “Third Test” : is often necessary, when no conclusion can be drawn after the card test i.e.. when no change in the response is noticed during answering “No” in connection with questions related to the card test. During the third test, the same questions are repeated after informing the person that, the polygraph showed that, he was not always truthful while answering the questions.

4. Mixed Question Test: to know whether the answers given for relevant questions during the first test, which have shown some changes in recorded response are correct or not, the person may be subjected to another test with just rearrangement the same questions. If the responses to the individual questions are same as during the first test then, it indicates that, the answers given are genuine.

5. “Yes” Test: in some persons, another test in a changed form may be necessary. Here, among the irrelevant questions some false incriminating questions are also asked. Before setting him for the test, he is narrated the questions and asked to replay in “Yes” in all questions. It is expected that there will be change in the response when he says “Yes” for a false incriminating question. If there is no change in response for the incriminating question then, absence of change in response against relevant questions in the previous test becomes meaningless.

6. “Guilt Complex Test” : in the guilt complex test, offence for which a person is interrogated is not the matter of questions directly. But questions which will create a guilt complex in him are asked. For example, if he had hit a person then, he may be asked question like, “You carried dagger with you when you met him that day” or “you have knowledge that the person was stabbed on that day at that time”.

7. “Re-examination Test” : Re-examination is carried in case of erratic response or with unresponsive persons or in persons with inconsistent behaviour. The examination is done by specific stimulation by general suggestive questions which does not bear an element to unnecessary excite the person. The question should be like. “You know that such and such person has been stabbed” 2 ; “you also can say who might have stabbed him”.

8. Peak of Tension Test — These questions contain the fact of offence without directly linking the person with the offence in a specific manner. For example, when it is a case of theft of some ornament, the interrogator may name several ornaments out of which one has been allegedly stolen by the person. The peak excitement period will be at the time of naming the exact ornament, which was stolen and that peak of excitement will be represented in the graphic records. It is suitable for persons who remains to some extent excited and alert all along the period of interrogation.

Of all the responses, the response in the change of respiratory function and in the GSR (Galvanic Skin reaction) are more dependable. The circulatory response represented by pulse rate and blood pressure may be helpful but often are less so. Further, variation in circulatory response may occur due to various reasons like, over consciousness, contraction of muscles, body movements etc.