56 



PHOTO-MICROGRAPHY 



Fig. 25 



marks are really strictly in focus. It is a good plan to draw a pencil mark on the 

 ground glass, and then applying the magnifier to its plain surface, to hold both to the 

 light, and to keep altering the focussing-screw of the magnifier until the grains of the 



lead are seen on the glass. No pains 

 should be spared in getting this focus 

 accurately, for, it is needless to point out, 

 if carelessly done, it will affect the final 

 use of the magnifier. This is the only 

 trouble of using- three kinds of g-round 

 glass, lest one should vary in thickness 

 and so upset the focussing arrangement of 

 the magnifier. Of course, the only way 

 out of such a trouble is to be careful to 

 procure three pieces of the same thick- 

 ness. 



With our arrangement, when we desire 

 to increase the camera length we employ 

 the "additional front" to which reference 

 has already been made. In Fig. 24a the 

 apparatus is used with no auxiliary lens, 

 no water bath, and no " front," but in b 

 these are all shown. 



Here ends the description of the usual 

 form of apparatus sold for the purposes of 

 photo-micrography ; but to make our 

 description as comprehensive as possible, 

 ii we must state that at times the photo- 

 - graphing of specimens which must be kept 

 horizontal demands the employment of 

 what is termed a "vertical arrangement." 

 A few workers in the subject speak highly of this form ; indeed, the celebrated Dr. Yan 

 Heurck prefers it entirely for all classes of work. This is not, however, we think, the 

 opinion generally met with. The form of stand, until quite recently, which has met 

 with the most approval is that designed by Dr. Van Heurck himself, and sold by 



