262 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
fluid used liad been prepared and sold by a prominent optician, and I 
had no thought but that its refractive index was what it should be. 
Another evening was given to the examination of the lens over the same 
diatom ; failure. A third evening was devoted to the same work, and 
failure was the reward. I then gave it up, and condemned the objective 
or my own skill, being disposed towards lack of confidence in the latter. 
Yet others had said that that objective would resolve that diatom. A 
fourth evening was given to it with the same result. Then it suddenly 
seemed stupid not to think to try another immersion fluid. There might 
be something lacking in this. I had cedar oil from a well-known 
European optician, and with a drop of it the objective was focused, 
with the light as oblique and the mirror exactly as before, when the 
lines on that shell stood out, if not like the pickets on the fence, at least 
with a sharpness, clearness, and neatness that was as delightful as it was 
amazing. In the twinkling of an eye the diatom was resolved to per- 
fection, while with the glycerin fluid, failure and discouragement had 
been the only results. The objective was vindicated, and so was any 
skill that the observer might, in a moment of self-complacency, imagine 
to be his. But on the table were two other glycerin fluids, one by 
a prominent and accomplished optician of this country, the other by 
a famous American, who is by all odds the equal of any optician in 
the world. The immersion fluid from the latter refused to have anything 
to do with those lines ; its action being similar to that of the com- 
position first tried. But the objective was not at fault, nor the adjust- 
ment. 
The other fluid was then tried, and the resolution was in every 
respect the equal of that made with cedar oil ; if anything it was 
superior. But there was as usual the fly iu the ointment. To remove 
the glycerin from the objective it is necessary to wash it off with 
water, but in this case, when the water drop was added, I had a 
moment of anxiety, for the fluid became white and opaque as milk, 
and I could see white particles falling on the lens front, like little 
flakes of snow. Investigation proved that the salt dissolved in the 
glycerin, a solution which makes so perfect an immersion medium, 
acts chemically on the nickel plating of the objective, and the glycerin 
seizing the water, allowed the new salt to fall in opaque white par- 
ticles. The chemical action is so great, that after using the medium 
for three times, there was deposited on the cover of the test-plate 
an iridescent film, having an irregularly circular outline, showing 
where the metal and the fluid had been in contact. Nor is this all, 
for across the surface of the front lens itself is a streak of the same 
insoluble iridescent deposit. The optician declines to make known 
the composition of the fluid, although he might reveal it with confi- 
dence, since no microscopist would ever make the medium for his 
own use after having a little experience with it. Its action on brass 
is similar to that on nickel, and must forbid its use as an immersion 
medium, although it is really the equal of the renowned cedar oil. To 
the latter, useful as it is, valid objections are its tendency to flow too 
freely, and the trouble needed to clean it from the lens, alcohol being 
demanded to remove it entirely, whereas with glycerin, a drop of water 
is enough. Cannot some of our opticians give us a glycerin medium 
