ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
289 
C5) Mounting-, including Slides, Preservative Fluids, &c. 
Can mounting’ media be improved for high powers by increasing 
the index of refraction?* — In answer to this question, Mr. J. D. Beck 
writes : — “ It has been the aim of the microscopist to increase the refrac- 
tive power of mounting media for diatoms, bacteria, biological and other 
specimens requiring a high amplification and the best resolution. 
Whether better results are attainable in this direction I am unable to 
say. All my diatoms, slides from J. D. Moller and others, are mounted 
dry or in balsam ; I have never tried Prof. Smith’s medium. If the increase 
in refraction is an improvement, would it not be a desideratum to attain 
still more satisfactory results, which perhaps might be accomplished by 
increasing the index of refraction of mounting media ? The desi- 
deratum is to see what exists, and to secure for that the most favourable 
means, bearing in mind that we must not expect too much from the best 
lenses under unfavourable conditions or circumstances. A certain 
quack condemned my Beck’s 1/6 in. objective because with it and a 
Beck’s No. 2 ocular he could not see bacteria in spring water, when in 
fact the water, which was cold as ice, came out of a mountain of rocks 
so free of vegetable and organic matter that no organisms could live in 
it, while a drop of water from a rivulet showed thousands of bacteria 
under the same lens. 
Insomuch as a large majority of microscopists cannot afford to buy 
the new Zeiss apochromatic objectives, they may perhaps increase the 
resolving or defining powers of the lenses of a cheap grade by im- 
proving the refractive properties of mounting media. While the philo- 
sophy of the Irishman, that “ if a little is good, more is better,” when 
he imbibed the second glass, may he rather extravagant in such cases, 
yet it may be solid philosophy for practical purposes in other directions ; 
so then, may we not continue to experiment on media to increase the 
refractive power until we find still more satisfactory results ? 
A friend of mine copied and sent me a list of refractive indices. 
The highest index of fifty substances given is that of chromate 
of lead at 2*50 to 2*97. It would appear that all the salts cf 
lead and zinc have a high index of refraction, which seems to be very 
much increased by the action of chromic acid, which probably exists in 
the metal chromium in a higher degree than in lead or zinc. I do not 
believe that nitre, which combines with chromium to form chromate of 
potassium, afterwards changed to bichromate of potassium through the 
action of sulphuric acid when exposed to acetate of lead, really increases 
the refraction of chromate of lead. I have my doubts whether the 
acetate of lead adds any refractive power to the bichromate of potassium. 
Native sulphur is given at 2*115, but when distilled with charcoal and 
reduced to a volatile spirit by adding one atom of carbon to two atoms 
of sulphur, forming bisulphide of carbon, the index is reduced from 
2*115 to 1*678. This is what the carbon has done, and yet diamond, 
which is carbon crystallized, is way up to 2*47 to 2*75. I suppose it 
would be impossible to bleach and to reduce the chromate of lead to a 
colourless medium without destroying its high refraction. We might 
expose colourless linseed oil to the action of chromate of lead by heat, 
* Microscope, x. (1891) pp. 18-20. 
1891. 
U 
