NOTES AND MEMORANDA. 
288 
and can only be ascertained by carefully studying tbe genesis of the 
animals.* 
Fluid Mounting, — At the April Meeting of the Microscopical 
Section of the Troy (U.S.) Scientific Association, the Eev. A. B. 
Hervey described a method, which he had recently devised. In 
his study of the Algae and Lichens he had been troubled, as others 
have been, by the difiiculty of permanently mounting specimens 
while studying them, without waste of time or change of arrange- 
ments. Most of the methods of mounting either ruin such 
objects entirely or else require considerable time, care, and S2)ecial 
appliances that are troublesome to a busy student, and therefore 
instructive specimens are lost. The objects may be transferred 
from water to Tarrant’s solution of gum and glycerine, and mounted 
without delay ; but the structure is not well preserved, and air 
bubbles are likely to be obstinately present. The objects show best 
in distilled water, sea-water, camphor-water, &c. ; and to mount them 
instantly and with uniform success he prepares cells of the gum and 
glycerine solution put on by means of the turn-table in the usual 
way. Having made cells of the required depth, and laid them aside 
until thoroughly dry, the inner half of the width of the cell is 
varnished on the turn-table with gold size, which is also allowed time 
to dry perfectly. Objects in water are arranged and covered in these 
cells with ease, and are ready after lying aside for a time varying 
from a few minutes to a few hours, to receive a coat of gold size or 
other varnish, the fiuid that exudes from the cell in pressing down the 
cover-glass having dissolved enough of the gum cell to hold the cover 
in position. It has not been found that the cell is too much affected 
by the fluid ; but if it should be so, the cell could be made of the usual 
cements, insoluble in water, and then coated with a thin layer of gum.f 
Influence of Temperature on the Optical Constants of Glass. — In an 
article in the ‘American Journal of Science and Arts’ (April), 
Mr. C. S. Hastings gives the results of some investigations which he 
has made on this subject. “ The most surprising fact,” he considers, 
“ which these results point out is that the variation in dispersive 
power attending variation in temj)erature is relatively enormously 
greater than that of the refractive power, a fact which has, he believes, 
escaped attention heretofore. It could hardly have escaped unheeded, 
however, did not a singular relation obtain in the coefiScients. 
The dispersive powers of three specimens of glass (flint, sp. gr. 
3*554; do. sp. gr. 3*151; crown, sp. gr. 2*482) computed in the 
ordinary way, are as 9 : 8 : 6 nearly, while the coefficients in question 
are as 9 ; 6 : 5 nearly ; hence if this relation holds approximately for 
all optical glasses, as is probable, an achromatic combination good for 
one temperature is good for all others within moderate limits.” 
Protecting Gap for Focussing under Water. — This, which was 
described in ‘ M. M. J.,’ vol. viii. p. 44, appears to have been recently 
re-invented, under the name of “ Dudgeon’s Submersion Cap.” 
* ‘Ann. and Mag. Nat. Hist.,’ ser. v. vol. ii. p. 251. 
t ‘ The American Naturalist,’ vol. xii. p. 333. 
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