14 THE A B C OF PHOTO - MICROGRAPHY 



a screw collar for adjusting its distance from 

 the plane of focusing screen. This costs but 

 fifty cents. A more elaborate and better 

 form, commonly known as Darlot's focusing- 

 glass, consists of a compound lens, mounted 

 in somewhat similar manner, with a clamping 

 screw for fixing the focus when adjusted. 



A much lower power than the f" objective 

 of these outfits is highly desirable, in fact 

 necessary, for photographing many large ob- 

 jects, opaque subjects, etc. A most useful 

 lens is a good 2 -inch properly corrected 

 for photography. Unfortunately, the limited 

 length of rack on all the students' microscopes 

 forbids the focusing of an objective of so low 

 a power unless it be in a very short mounting, 

 which is not possible for a well-corrected 

 doublet. A feature of the Spencer microscope 

 will be found most useful and essential in this 

 case. The lower end of draw-tube is fitted 

 with the society screw, to which the objective 

 may be attached and used within the com- 

 pound body, thus enabling one to utilize the 

 very lowest powers with a small and compact 

 stand. 



I have spoken of the 2" objective being 

 corrected for photography. It is not perhaps 

 generally known that the corrections of the 



