PRACTICAL PHOTO-MICROGRAPHY. 365 



the focal length in mm. of the objective. Example : With an 

 objective of 3 mm. focus, a "No. 3 projection eye-piece and a 

 screen thirty inches from the shoulder of the ocular, we get a 

 magnification of approximately 750 diameters, 30 inches = 

 say, 750 millimeters. 



750 X 3 (ocular) = m 



3 (focus of lens) 



But this applies with only moderate accuracy when we have, 

 lor instance, racked our tube to 10J inches in place of the 250 

 mm. (10 inches) for which the ocular and objective are in- 

 tended. If, however, our tube length be really 250 mm. our 

 magnification may be taken as almost exactly 750 diars. in the 

 above given example, at least the writer has not been able on 

 experiment to verify any inaccuracy in the figures given. 



The future advance in photo-micrography if there is to be 

 any advance in the optical line will depend upon apochro- 

 matic objectives and condensers, and the use of wide angles. 

 It is vain to say that all the greatest discoveries have been 

 made with low-angled glasses, though the statement may be 

 perfectly true. Had higher angled glasses been used the dis- 

 coveries would have been made all the sooner, and our high- 

 angled glasses of to-day demonstrate with perfect ease even in 

 unskilled hands what required years of study and the most 

 skilled microscopists to certify in by-gone days. And further, 

 the science of practical optics was, say twenty years ago, far 

 behind where it is now, and the opticians of these days, in 

 achieving high angles, probably introduced such errors of cor- 

 rection as made the glasses practically worthless. So the ob- 

 servers did well to use well corrected low-angled glasses 

 rather than faulty high-angled ones, and their discoveries 

 were made with low-angled glasses, faute de mieux. The 

 writer is in the constant habit of examining numbers of 

 objects of the most diverse kinds, he has at command 

 high-angled and low-angled glasses, yet even for cursory 

 examination of such subjects as pathological, physiological, 

 bacteriological and diatornaceous objects, he invariably takes 

 as if by instinct the widest angled glass he can find, of suitable 



