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BELL SYSTEM TECHNICAL JOURNAL 



of the specimen show little contrast, either within the specimen itself, 

 or between the specimen .and the background. Therefore, to re- 

 produce an object faithfully or to accentuate faintly revealed char- 

 acteristics, careful consideration must be given to the color of the 

 light used when photographing the specimen. For the purpose of 

 separating white light into well defined bands, light filters are used 

 and their function is to filter out rays or bands of rays of certain 

 given wave-lengths. These filters consist of colored gelatine films 

 mounted between Hat pieces of glass or of litiuids appropriately 

 colored and contained in rectangular vessels of glass with flat and 



Fig. *) — llilgc-r wiivf-length spcftrometer. The camera is inU-rchangeaMy mounted 

 with n reading telescope. 



parallel side walls. Tin- "W'ratten M" series of gelatine and glass 

 tillers is |)rol)al)ly the best known and most wiileh' used. The selec- 

 tion of a light filter for a given specimen is usually by experimental 

 methods. Successive filters are inserted in the illmninating beam and 

 the resulting image stiiiliiil fni- rendering and definition. However, two 

 simple rules apjily generally; if a color is to be rendered as black as 

 jjossible, then it must be jihotographed by light of wave-lengths within 

 the alisorption bind of the sjiecimen; when contrast is desired within 

 the sjMjcimen itsill'. ilie object should be photograi)hed by light of a 

 wave-length which il transmits. The first rule is of use when it is 

 desired to secure contrast between the object and the background ; and 

 the second for better rendering of detail within the object. 



S|)ectrometcrs are axailabU- for determining the characteristics 

 of filters; for determining the tr;iiisniissioii s(iectriMii of a inicro- 



