2i 4 PHOTO-MICROGRAPHY ' \_CH. VIII 



inclined, and fastened to the fixed part of the frame supporting the 

 camera. As the stage of this microscope is moved by the coarse or 

 the fine adjustment, the focusing can be accomplished with the same 

 accuracy as the microscope itself. For the general arrangement of 

 the specimen and the rough focusing the ground glass is used, then 

 this is replaced by a clear-glass focusing screen, and by the aid of a 

 focusing glass the specimen is put in perfect focus. (See also § 282.) 

 As one cannot reach the fine adjustment while focusing, the well 

 known device of a cord over the head of the micrometer screw is 

 resorted to. The two ends of the cord should be weighted with 

 about 50 or a hundred grams to keep the cord taut, then whichever 

 one is pulled, the micrometer screw will respond at once. To cut 

 off the light a piece of black velveteen is hung over the end of the 

 objective. This can be removed without jarring the apparatus. 

 An exposure of a few seconds (3 to 10 seconds), will suffice for 

 many preparations, unless a color screen is used. The color screen 

 increases the time of exposure (§ 291). 



COLOR-CORRECT PHOTOGRAPHY 



From the fact that the different wave lengths of light affect the photo- 

 graphic plate with different degrees of vigor, the ordinary photographic print 

 of many-colored objects or landscapes is not satisfactory. All objects whose 

 light is of short wave lengths, as blue, etc., will appear too light and those 

 with greater wave lengths as red, yellow and green will be too dark relatively. 

 To obviate this difficulty two methods have been adopted, and for the most 

 complete success they must be combined. 



(A) The use of ortho- or iso-chromatic plates and (B) the use of a color 

 screen or light filter. 



\ 290. Orthochromatic or Isochromatic Plates. — These are plates which 

 have been rendered much more sensitive than ordinary plates to the long 

 waves of red, orange, yellow and green, they therefore give a much more 

 natural rendering to many-colored objects than ordinary plates. While color- 

 sensitizing has been carried to a considerable stage of perfection and there is 

 a large choice of plates now on the market for special purposes, it should be 

 remembered that no matter for what color or colors a plate has been sensitized 

 it remains more sensitive to the short waves (violet end of spectrum) than to 

 the long waves (red end of spectrum). Therefore to obtain a correct rendering 

 of variously colored objects by photography, a color screen is necessary as 

 well as a color sensitive plate ( \ 291) . 



These color-correct plates are not very enduring, and must be used while 

 they are fresh, or only weak, foggy negatives will result; and as they are sensi- 

 tive to orange, etc., one must be very careful in exposing them in the dark 



