356 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



covering the whole spectrum. By using colored screens it is thus pos- 

 sible to take the colors out of an object; the slightest cancellation 

 mark on a stamp that has already been used is then clearly revealed. 

 Here again, advances in physics have affected criminal investigation. 



We are now able to make screens by which the 2,500- Angstrom 

 band of mercury in the ultraviolet can be isolated. Photographs of 

 objects taken in this light reveal certain details which are otherwise 

 invisible. As regards infrared light, the electronic telescope — an im- 

 provement upon the image-converting cell developed during the war 

 to enable fighting forces to see at nighttime their opponents lit up 

 by infrared light which is invisible to the human eye — has quite 

 recently been adapted in Belgium for use in forensic science 

 investigation. 



Some objects whose surface needs to be photographed reflect light 

 so strongly that it is impossible to photograph them directly. This 

 applies to carbon papers that have been used in typing a document. 

 Their shiny surface is dulled where the characters of the machine 

 have struck the paper. When we try to photograph them, however, 

 we find it impossible because of the reflection, particularly as they are 

 veiy often crumpled. It is necessary to incline the surface at an angle 

 to the axis of the lens so that the reflection is eliminated. The result- 

 ing photographs are distorted, however, and the original can be recon- 

 structed only if we know the angle of inclination of the document; 

 incidentally, the angle cannot be chosen haphazardly but must be 

 calculated mathematically beforehand. 



The trichromatic selection of colors on which color printing is based 

 is far from perfect. It is very often impossible to cancel out a color 

 completely, especially when it contains white or black, which is gen- 

 erally the case. It is still less possible to eliminate black characters 

 on a colored background by photographic means, but it can in fact be 

 done hj the process described as "superimposing positives and nega- 

 tives," as the following example shows. 



A registered letter is handed in to a post office bearing stamps 

 already canceled but with the postmarks almost invisible. In order 

 to insure that these marks are not discovered, the post office clerk, 

 who is involved in the fraud, stamps several times over each of the 

 stamps. How can we discover whether, when the envelope was put 

 into the post, 1 he stamps were or were not already postmarked ? The 

 colors of the stamps can easily be screened out by a simple selection 

 of colors, leaving only the postmarks; but, even in enlargements, it is 

 impossible to discern whether or not there are other marks from dif- 

 ferent stampings among the stamp marks of the dispatching office. 

 In such a case, we may photograph on transparent films : (a) the stamp 

 of the dispatching office as a positive; (b) the postmarks on the stamps 



