SCIENTIFIC DETECTION OF CRIME — SANNIE 355 



examination shows, however, that the clay of which it is made is a 

 coarse chiy containing much more iron than the cLays used in making 

 artistic terracottas. The pigments used are indeed the same as those 

 employed by the fifteenth-century artists, but the same ones are still 

 used today. The patina is artificial, being produced by a varnish with 

 a basis of linseed oil, ochre, and verdigris. Finally, the panel has been 

 broken and restored, but the restoration was done when it was still 

 new and was strengthened by iron bars, one being made of rolled iron. 

 But rolled iron dates from 1850 and the panel is therefore a recent 

 forgery. 



The works of art most often forged are paintings, and a wide va- 

 riety of photographic and radiographic methods have to be used in 

 their expert examination. Repainting is shown up by ultraviolet or 

 infrared light ; X-ray photographs, for which rays of low penetrating 

 power are used, are a still better means of discovering traces of earlier 

 painting beneath a later layer. Pigments can be identified by micro- 

 chemical and spectrographic analysis and, as we know the dates at 

 which they were introduced and ceased to be used in the last 10 cen- 

 turies, they give us an invaluable means of dating certain works of art. 



Arson is also difficult to detect. It is, as may be imagined, extremelj' 

 hard to discover the substance left by the criminal in a mass of cinders 

 and charred fragments, especially as such substances are highly in- 

 flammable. There may, for instance, be phosphates resulting from 

 the combustion of a phosphorus bomb, the ashes of straw used to light 

 the fire, or traces of oil or petrol scattered about the premises. All 

 the processes of chemical analysis are then called into play, the main 

 difficulty being to isolate the incendiary material from the ordinary 

 residue of combustion. 



Optical methods. — Scientific detection was, at the outset, a science 

 of observation, supplemented by photography. Bullets and cartridge 

 cases can thus be identified, footprints, signs of housebreaking, and 

 tire marks compared, and forgeries on documents, identity cards, 

 postage stamps, etc., detected. Every improvement in these photo- 

 graphic methods means a step forward. One of the most spectacular 

 dates from the use of the Wood lamp, already mentioned, which pro- 

 vokes fluorescence in a large number of substances. Under the Wood 

 lamp, obliterations in manuscripts are revealed, as well as writing in 

 invisible ink, and gums and waxes of differing compositions can easily 

 be distinguished. The colors of counterfeit postage stamps, or even 

 the gum applied to them, can be distinguished from genuine colors 

 or gums. Infrared photography also often shows up things that are 

 invisible to the eye, such as the scorch marks around a hole caused by 

 a shot fired through a piece of black clothing at close range. 



About 20 years ago, manufacturers of photographic plates began 

 to produce emulsions of varying degrees of chromatic sensitivity, 



