SCIENCE IN CRIME DETECTION HOOVER 221 



ment actually used, by virtue of the microscopic marks which 

 small dents and other imperfections in its cutting edges leave upon 

 the severed ends of the window bars. 



Case after case could be cited, each with a different story to tell, 

 illustrative of the endless manner in which applied optics in the 

 visible region of the spectrum has yielded a welcome solution to 

 an otherwise difficult problem. However, space will not permit a 

 more detailed discussion. 



Corresponding to ultraviolet light lying beyond the short wave 

 length at the end of the visible spectrum, there appears above the 

 limit of visibility at the long wave length end, light which is known 

 as infrared. This portion of the spectrum also has been found to 

 possess certain characteristics which are of value to the scientific 

 investigator; chief among these is the ability of infrared radiation 

 to penetrate materials which are normally opaque to the unaided 

 eye. As an example of this property, plate 4, figure 1, shows a 

 photograph by ordinary process of an obliterated return address 

 appearing upon an envelope in which an anonymous derogatory 

 letter was mailed. It will be noted that the obliteration has been 

 carried out by marking over the original form with ink. Aware of 

 the characteristics of the infrared portion of the spectrum, the 

 examiner immediately photographed the questioned area utilizing 

 suitable optical filters and special photographic plates sensitized to 

 this type of radiation. Plate 4, figure 2, shows the resulting pho- 

 tograph, clearly revealing the original printed return address 

 through the overlayer of ink which now appears only as a light 

 smudge. Here again by properly applying familiar principles of 

 selective optical absorption, the source of the stationery employed 

 by the anonymous writer was readily ascertained. Without the 

 utilization of infrared photography much time and effort might 

 have been required to accomplish the same end. 



Another instance involves the examination of a leather money 

 bag found on the person of a desperate criminal following his 

 arrest. Other equipment in the possession of this criminal included 

 24 sticks of dynamite, a supply of nitroglycerin, numerous travelers 

 checks, and a supply of weapons. 



Visual examination of the money bag, as indicated in plate 5, 

 figure 1, disclosed no identifying data which would be of assistance 

 in tracing it. However, upon its receipt in the technical laboratory 

 of the Bureau, infrared photographs clearly disclosed the name 

 of the bank where it had originated, as shown in plate 5, figure 2. 



Moving to the long wave length portion of the radiant energy 

 spectrum, the contribution of radio to law enforcement must be 

 acknowledged. The rapidity with which radio as a means of com- 



