224 PHOTOGRAPHING WITH THE MICROSCOPE [Cn. VII 



clearness. In such a case the part of the section showing details 

 satisfactorily should be surrounded by a delicate ring by means of a 

 marker (fig. 59-61). If one's preparations have been carefully studied 

 and the special points in them thus indicated, they will be found far 

 more valuable both for ordinary demonstration and for photography. 

 The amount of time saved by marking one's specimens can hardly 

 be overestimated. The most satisfactory material for making the 

 rings is shellac colored with lampblack. 



Formerly many histologic preparations could not be satisfactorily 

 photographed. Now with improved section cutters, better staining 

 and mounting methods, and with the color screens and isochromatic 

 and panchromatic plates ( 380) almost any preparation which shows 

 the elements 'clearly when looking into the microscope can be satis- 

 factorily photographed. Good photographs cannot, however, be 

 obtained from poor preparations. 



In photo-micrography do not forget the three ways in which details 

 of structure may be brought out clearly: 



(1) By difference of refraction of the object and the mounting 

 medium (refraction images, 137). 



(2) By differential staining (color images, 139). 



(3) By means of dark-ground illumination ( 117). 



EXPERIMENTS IN PHOTO-MICROGRAPHY 



347. The following experiments are introduced to show practi- 

 cally just how one would proceed to make photo-micrographs with 

 various powers, and be reasonably certain of fair success. If one 

 consults prints or the published figures made directly from photo- 

 micrographs it will be seen that, excepting diatoms and bacteria, 

 the magnification ranges mostly between 10 and 150 diameters. 



348. Focusing in photo-micrography. For rough focusing and 

 as a guide for the proper arrangement of the object one uses a ground- 

 glass screen, as in gross photography. With the ground-glass screen 

 one can judge of the brilliancy and evenness of the illumination more 

 accurately than in any other way. For final and exact focusing two 

 principal methods are employed: 



(a). A focusing glass is used either with a clear screen or in a 



