ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



119 



If the projected image is exactly focused, it ought to be seen with the 

 lens at the same time with the fine lines traced upon the glass. In this 



Fig. 23. 



apparatus the size of the plates is 12 cm. by 16 cm., a size which is 

 recognized as sufficient by all who have had experience in photo- 

 micrography. 



Nelson's Photomicrographic Focusing Screen.* — Mr. G. Smith, in 

 reference to Mr. Nelson's suggestion f for ruling the focusing screen 

 with metrical and English scales, considers that if diamond lines are 

 used they should be ruled horizontally and vertically about ^ in. apart ; 

 but better still, every third line should be missed. The cross ruling 

 thus forms a kind of plaid pattern, and any decided pattern materially 

 assists the eye in keeping to the proper plane instead of seeking a focus 

 on either side. The eye-piece must of course be first adjusted exactly 

 to these lines for the operator's eye, 



Another very simple and effective plan (applicable to other cameras 

 too) is to rule diagonals in blacklead pencil across the ground glass, and 

 over the centre cement a thin cover-glass, taking care to put there a few 

 grains of dust, or say, cotton fibre. Both these plans he has used for 

 many years, and can recommend both ; with either it is easy to focus the 

 darkest interior. 



Neuhatss, R.— Anleitung zur Mikrophotographie fur Aerzte, Botaniker, &c. 

 (Guide to Photomicrography for Physicians, Botanists, &c.) 



32 pp., 8vo, Berlin, 1887. 

 Sternberg. G. M. — Photo-micrography in Medecine. 



Reference Handbook of the Medical Sciences {U.S.A.) 1887, pp. 647-58 (7 figs.). 



(5) Microscopical Optics and Manipulation- 



Histological Structures and the Diffraction Theory. — Hitherto the 

 examples of the action of diffraction in microscopical vision have been 

 almost entirely confined to diatoms, objects which more than any others 

 are suited to illustrate the principles on which the theory is founded, 



Eug. Mech., xlvi. (1887) p. 394. 



t See this Journal, 1887, p. 1028. 



