206 



PHO TO-MICROGRA PH ) ' 



\_CH. VIII 



ground glass for various sizes may be determined once for all, and these posi- 

 tions noted in some way. 



In the camera here figured, the camera bed is ruled in centimeters so that 

 the position of the ground glass can be determined with accuracy and noted. 

 It takes but a moment to set the ground glass or focusing screen at the right 

 level to give any desired size. In practice it is convenient to have attached to 

 the camera a table giving the position of the ground glass for various sizes, 



Fig. 162 Beck's Isosligmar Objective. " The lenses are ground u<ith 

 shallow curvature and mounted with air spaces instead of cement between the 

 individual elements"' 1 (Cut loaned by Williams, Brown & Earle, Phila). 



and also the distance of the objective from the object in each case, By having 

 this information it takes but a moment to set the camera and to place it so 

 that it will be approximately in focus. The final focusing is then accomplished 

 by the use of the rack and pinion movement. It is an advantage to use a 

 focusing glass and a clear focusing screen or the transparent part of the ordi- 

 nary screen (Fig. 163), for the final focusing. Since many objects have no 

 sharp details which one can focus on, it is helpful to focus on some printed 

 letters put on the part to be brought out with the greatest sharpness. Of 

 course these are removed before the exposure is made. 



Fig. 163. Ground-glass focusing 

 screen u'ith central transparent area for 

 exact focusing ivith a focusing glass when 

 one does not possess a clear focusing screen. 

 (/) The groimd surface; (2} Central 

 part with oblong cover-glass over Canada 

 balsam on the ground surface to render it 

 transparent. X. The central point in 

 the entire focusing screen; It ^s made 

 with a black lead pencil on the ground 

 surface. The focusing glass is focused on 

 this cross, then when the image is in focus 

 it will be at the level of the sensitive 

 coating of the plate. 



\ 274. In Lighting the object one should take pains to so arrange it with 

 reference to the light that the details will show with the greatest clearness. 



