ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



531 



the lower membrane are unmistakably below the intercostals of the 

 upper membrane, and that this is true not only of P. migulatum but 

 also of all allied forms that have been examined. 



Fluor-Crown, a New Optical Glass.* — E. Zschimmer describes how, 

 by the use of fluor-spar as an ingredient, certain durable glasses suitable 

 for optical work have been obtained. They are numbered respectively 

 in the Jena Glass Catalogue : 6781, G500, 7185. Some of their 

 constants are respectively : — 



n^ = 1-4938, 1- 471(1, 1*46:37 ; 



mean |^ o 00706, 0' 00701, 0-00707 ; 

 dispersion j 



V = 69-9, 67-2, 65-6. 



The author is of opinion that further advances may be still possible. 



Microscope Construction and the Side-screw Fine-adjustment.t 

 E. M. Nelson summarizes the history of the side-screw adjustment, and 

 points out that in modern times horizontal fine-adjustments may be 

 placed in two groups, viz. {a) those with continuous motion, and (h) 

 those without. "The drawback which those of the first kind possess is 

 that the user does not know whether he is focusing up or down ; and 



Fig. 75. 



Fig. 76. 



the drawback which all the second kind (excepting the Berger) have, is 

 that of damage and injury to the delicate moving parts when they butt 

 up against a stop. The Berger avoids all risk of damage from this 

 source bv causing an idle nut to butt against a stop : if this nut receives 

 damage "or strain to its thread it is of no importance. The first kind 

 adopt a continuous motion in order to secure immunity from this 

 danger, and put up with the great disadvantage of having a fine adjust- 



* Zeitschr. f. Instrumentenk. xxxiii. (1913) pp. 145-8. 

 t Journ. Quekett Micr. Club, xii. (1913) pp. 96-8 (2 figs.). 



N 2 



