ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



665 



Mr. T. Curties also devised a method of mounting the disk for use 

 in the ordinary substage (Fig. 150 — h scale)c A rod is made to slide 

 in a small spring-tube in a substage fitting ; and the disk is fixed to a 

 short pin attached to an angle-piece at the top of the rod for conve- 

 nience of centering, &c. 



Fig. 150. 



Fig. 149. 



At a meeting of the Liverpool Microscopical Society last year, a 

 very simple plan of mounting the disk was exhibited. A section of cork, 

 about half an inch in thickness, was fitted to the stage opening beneath ; 

 and an aperture was cut in the cork suitably to hold the disk at the 

 proper level and to allow free incidence of light. 



Smith's "V-shaped Diaphragm." — This was suggested by 

 Dr. J. Edwards Smith in 1875 as of singular advantage in resolving 

 severe tests, and he now says * that although in the interim a large 

 variety of substage illuminators have been brought out, he still prefers 

 it for " a clean square resolution of severe tests by oblique light." It 

 consists simply of a piece of japanned iron plate (the ordinary " ferro- 

 type " plate used by photographers) of say 3 inches square, which is 

 fastened to the under side of the stage by one or more screws near 

 its edge farthest from the mirror or lamp, and then bent down as 

 shown in Fig. 151, the open side being adjacent to the source of illu- 

 mination. 



Dr. J. E. Blackham also records f his experience of the diaphragm 

 for high angles, first pointing out that the name, V-shaped, is not 

 well chosen, as the diaphragm itself is not V-shaped, but only forms 

 one side of the V, the under side of the stage forming the other. 

 " Of course, if the ferrotype plate is flat, it simply closes the well- 



* Amer. Journ. Micr., vi. (1881) p. 59. 

 t Ibid., pp. 9-10 (3 figs.). 



