262 
CORRESPONDENCE. 
millimeter, exhibit equal distinctness, and bear a magnification of 3500 
diameters with clearness and sharpness, and show the dots on Amphi- 
pleura pellucida, Surirella gemma and Grammatophora suhtilissima eveii 
without oblique illumination, but with oblique light, with great pro- 
minence, and the most delicate organic objects superbly." So much 
for the man's own account of the matter. I may now state what I 
have done with it myself. I tried it with respect to the several 
points of screw-collar, resolution, penetration, depth of eye-piece, and 
definition, with the following results : 
(1) Screw-collar. — So far as I have been able to examine it on this 
point, — and I tried it on covering glass ranging from • 003 to * 008, — 
the maker is fully justified in his statement. I found no difference 
whatever. 
(2) Besolution. — It worked upwards, with comparative ease, through 
S. gemma, P. macrum, Frustulia Saxonica, and Navicula crassinervis, 
resolving them with great beauty and sharpness ; but at Stauroneis 
spicula it STOPPED ; and no coaxing could draw it onwards. On this 
occasion I used a 0 eye-piece with oblique light from an ordinary 
concave mirror, l^ext night, having substituted a peculiar arrange- 
ment of my Abraham's prism, and again using the C eye-piece, the 
resolution was absolutely perfect from end to end, and the object as 
colourless as water. In both cases I used a Beckett lamp. Its reso- 
lution, then, I must pronounce excellent. 
(3) Penetration. — In this respect also it was perfectly satisfactory, 
showing layer after layer of tissue to a good depth ; and I can only 
conceive its failure where the operator cuts his sections too thick. 
(4) Depth of Eye-piece. — In this trial I had at command B and C 
eye-pieces by Baker, I) by Powell and Lealand, and E and F by Ross. 
The third of these 1 was soon obliged to discard, as it gave much in- 
ferior images to those presented by the E and F eye-pieces. 
On P. angulatum, with E eye-piece, the resolution was only 
moderately good, and there was a certain amount of unmistakable 
" fuzziness," which was not . pleasant. On S. gemma with E and F 
eye-pieces, the result was simply nil, though it had but a minute 
before resolved this diatom beautifully with a C eye-piece. 
In my own practice I should never think of using it with any 
higher eye-piece than 0. I also came to the conclusion that there was 
ample room for improvement in our eye-pieces. It will be seen, then, 
that I have come far short of doing with it all that the maker pro- 
mised it should do. This the reader may, if he likes, attribute to 
my want of manipulative skill. But there is something also in the 
fact that this glass, — confessedly one of a novel and singular con- 
struction, — had only been in my hands some thirty-six hours, — too 
short a time to enable one to become acquainted with all its little 
whims and peculiarities. For lenses too have their little whims as 
well as human beings ; and these have to be studied, and humoured, 
if one is to succeed in making a lens do its very best. 
(5) Definition. — This I conceive to be the distinctive feature and 
special excellence of Hasert's new system. Indeed, I seem to myself 
