CHAPTER VIII 

 LIGHT FILTERS AND SCREENS 



Plane Glass for Filters. — It is evident that to ensure 

 the maximum concentration of Ught at all points of the 

 object-field, the glasses through which the light passes 

 must be fairly optically correct. In using color filters, 

 additional errors need not be introduced by using coarse 

 glass. Hence the Wratten gelatin-film hght filters should 

 be cemented in fairly good plate glass, such as C of the 

 Eastman Kodak Company. Lantern slide covers have 

 been recommended for this. Pieces 1^ inch square are 

 large enough for the usual condenser with large lenses. 

 They can be supported in a simple holder, at right angles 

 to the incident light, close in front of {i.e., nearer the 

 light than) the mirror (or reflecting prism) of the micro- 

 scope. In cementing the gelatin films between pieces 

 of plate glass, film and glass are cleaned with xylol and 

 smeared with xylol balsam. Then they are pressed 

 together. If heated much to dry the balsam, the gelatin 

 may crumple up. 



Yellow-green Light Filters. — The yellow-green Wratten 

 series, Nos. 66, 56, 57A, 58 and 61, affords a graduated 

 sequence of amount of absorption; and the members can 

 be used, singly or combined, for regulating the intensity 

 of the light. The writer has needed no other regulator. 

 These same filters can be used for photography, with ortho- 

 chromatic plates; especially if the object is stained red, 

 as with carmine. The yellow-green filters (Nos. 66, 56, 

 or 57A, or two of them combined) can be used constantly 

 on the binocular microscope, during several hours each 

 day, with restful effects on the eyes. Numbers 58 and 

 61 are more suited to the monocular. (A blue-green 

 filter is preferred by some, but appears in time to tire 



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