248 Transactions of the Boyal Microscopical Society. 
found, in 1875, gave a bright field when used on balsam-mounted 
objects, provided they were examined with immersion objectives of 
sufficient angle. Mr. Wells gave an account of this method in the 
‘ Boston Journal of Chemistry,’ * in which he says : — “ In examin- 
ing Holler’s Probe-Platte, a balsam mount, under these conditions, 
with light from a kerosene hand-lamp, I easily resolved the 
Amphipleura pellucida, so clear and decided were the lines that 
with a power of 8000 they were still visible. . . . The resolution 
of this difficult diatom, as well as the Frustulia Saxonica and 
Nitzschia curvula (Nos. 18 and 19 on the Probe-Platte), far sur- 
passes any that I have ever seen by artificial light, and rivals the 
beautiful resolution obtained by monochromatic sunlight.” The 
paper of Mr. Wells was copied or abstracted in several of our 
journals, and the method has ever since been in constant use by a 
number of our microscopists. I note that it has just been redis- 
covered by Mr. Adolf Schulze, of Glasgow,! who, although 
apparently unacquainted with the American publication of this 
method, has arrived at results identical with those of Mr. Wells, as, 
indeed, all will do who patiently try it with suitable objectives. 
I do not claim for my own device, figured in this paper, that it 
gives better results than the best that can be obtained in this way, 
or with a suitable hemispherical lens or semi-cylinder, brought into 
immersion contact beneath the balsam-mounted object ; but it is 
not only cheaper than any of the plans hitherto described, but, as 
I think, and as all to whom I have shown it seem to find, it is 
much easier to use so as to get the best results with a fully illumi- 
nated field. 
In conclusion, I may remark that while it is clearly necessary 
to use immersion condensers to secure the greatest obliquity of 
illumination that can be admitted by the widest-angled modern 
immersion objectives, and while this great obliquity is of sub- 
stantial advantage in the resolution of lined test-objects mounted in 
balsam, it by no means follows that such condensers are necessary 
to the advantageous use of immersion objectives of more than 82° 
or even of 90° balsam angle, with central light, provided the object 
is mounted in Canada balsam. The minute details of thin sec- 
tions of normal and pathological tissues thus mounted and illu- 
minated, are far better displayed by such objectives than by those 
of inferior angle ; and this easily observed fact is so fully in accord 
with elementary optical theory that its discussion in this place 
seems quite unnecessary. 
* June, 1875, p. 140. 
t “An Easy and Simple Method of Besolvinp: the Finest-lined Balsamed 
Diatomaceoiis Tests by transmitted Lamplight, with special reference to AmpM- 
pleura pelluc'da” ‘ Journal of the Koyal Mic. Soc.,’ May, 1878, p. 45. 
