The Microscope. 19 



cumstances. A certain quack condemned my Beck's ^ inch ob- 

 jective 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 as 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 bac- 

 teria under the same lens. 



Inasmuch as a large majority of microscopists cannot afford 

 to buy the new Zeiss apochromatic objectives, they may per- 

 haps increase the resolving or defining powers of their lenses of 

 a cheap grade by improving the refractive properties of mount- 

 ing media. While the philosophy of the Irishman, " That if a 

 little is good, more is better," when he imbibed the second glass, 

 may be rather extravagant in such cases, yet it may be solid 

 philosophy for practical purposes in other directions ; ^ then 

 may we not continue to experiment on media to increase the re- 

 fractive power until we find still more satisfactory results ? 



A friend of mine copied and sent me a list from a table of re- 

 fractive 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 of lead and zinc have a high, index of refraction, 

 which seems to be very much increased by the action of the chro- 

 mic 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, after- 

 wards changed to bi-chromate of potassium through the action 

 of sulphuric acid when exposed to acetate of lead, really in- 

 creases the refraction of chromate of lead. I have my doubts 

 whether the acetate of lead adds any refractive power to the bi- 

 <;hromate 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 

 bi-sulphide 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 

 ■colorless medium without destroying its high refraction. We 

 might expose colorless linseed oil to the action of chromate of 

 lead by heat, and when well settled filter a number of times or 

 olarify it as varnish is clarified. This would become a rapid 



