CORRESPONDENCE. 
261 
himself was to construct an objective which should completely satisfy 
the conflicting requirements of scientific microscopists and of genuine 
' Naturforscher,' — which should be so absolutely perfect in its correc- 
tion as to dispense with screw-collar adjustment for different thick- 
nesses of cover, so as to work through a wide range of covering glass 
without any perceptible deterioration of the optical image, and com- 
bine the utmost beauty of definition with a depth of penetration 
amply sufficient for all medical purposes, and, while giving a perfect 
resolution of all known tests, including even those terrible puzzlers 
Stauroneis spicula * and Ampliijpleura peUucida^ be capable of being 
worked up by deep eye-pieces to 3500 diameters without detriment to 
the definition. 
In his correspondence he says : " I hope to bring my new lenses 
to such a point of perfection that they will not want any correction- 
screw for different thicknesses of glass cover. I tried a power of 
2000 diameters on very fine organic objects with covering glass of 
^ and of a millimeter, side by side, without being able to find any 
difference in distinctness. I also tried GrammatopJiora suhtilissima 
with cover of ^ and .-^^ without perceiving any difference." .... 
" My old dry lenses show longitudinal dotted lines on AmpMpleura 
pellucida, and lateral of the same kind ; and in very bright light the 
little corpusculi can be seen standing diagonally on the longitudinal 
lines, being of a rectangular form,| as you will find them on P. halti- 
cum and P. attenuatum. My new immersion lenses will resolve the 
object with ease, even with direct light, as they will also show the 
little corpusculi on the lines of Gram, suhtilissima and S. gemma.'" 
. ..." It [the new pattern immersion lens] will, even with direct 
light, resolve Frustulia Saxonica and the eeal Grammatophora suh- 
tilissima, which has never been resolved by any instrument which I 
compared with mine, neither English, German, nor French, all of 
which I compared at various times. But my slide of Grammatophora 
(in Canada balsam) was never resolved by any glass of the best 
makers. You will also see the puncta of ArapMpleura pellucid a with 
direct light." 
And in his recent catalogue, which appeared within the last three 
weeks, this new species of lens is thus described : " Objectives of 
newest construction, on the immersion system, requiring no correc- 
tion-adjustment for different thicknesses of glass, which, at a magni- 
fication of 2000 diameters, and on covering glass from to of a 
* A veteran microscopist of great skill as a manipulator, in liis letter to me 
dated Sept. 27, 1875, says: I have this morning succeeded in resolving Stauroneis 
spicula to my satisfaction. I have had a great deal of trouble witli it. It is 
undoubtedly the very hardest object I ever had to deal v^ith.,^ Amph. pellacida not 
excepted. I could, however, do nothing with St. spicula by lamplight. I soon 
found morning was the only light that would enable me to deal with it ; and I 
have now seen it resolved— I may say ' brilliantly,'— all over the loricay Com- 
pare also Mr. Leifchild's letter in the ' M. M. J.,' vol. xiii. p. 174. 
t Without professing to follow the learned Professor in his views of the mark- 
ings of Amph. pellacida, I may mention that a notable microscopist of my acquaint- 
ance sent him an especially difficult slide of that diatom, with a request tliat he 
would resolve it, and received in a few days a pencil drawing of it, corresponding 
in all its details with the above description. This drawing I have seen. 
