210 Miscellanies. 



Edinburgh, has actually obtained this result, by using a certain liquid 

 in place of one of the kinds of glass. 



The achromatic object-glass is usually composed of a convex crown 

 glass lens and a concavo-convex flint glass lens, of such curvatures 

 that the crown lens will produce a refraction greater than the flint 

 lens, sufficient to produce a positive focus at the proper point, while 

 the dispersions of the two lenses are equal. These conditions require 

 the refractive power of the flint glass to be considerably greater than 

 that of the crown glass, and the dispersive power of the flint to be 

 still greater in proportion to that of the crown, than the refractive 

 power. This requisite property of flint glass is produced by the ox- 

 ide of lead, which probably increases the dispersive power, nearly in 

 the duplicate proportion of the quantities added to the other materials. 



Having now stated the premises, as far as my limited knowledge 

 extends, I beg leave to submit the following inquiries : 



1. If we add a small quantity of lead to the materials of crown 

 glass, so as to answer the purpose of a common crown glass lens of 

 an object-glass, and also add a larger portion of lead to the same ma- 

 terials, so as to answer the purpose of the common flint glass lens, 

 will not these two kinds o{ glass have the same character, and pro- 

 duce spectra in which the several colors will be proportional, each to 

 each ? 



3. If we add a very small quantity of lead to the other materials of 

 flint glass, so as to answer the purpose of the crown glass lens, and 

 also add a larger portion of lead to the same materials, so as to an- 

 swer the purpose of the common flint glass lens, will not these two 

 kinds of flint glass have the same character, and produce spectra in 

 which the several colors will be proportional, each to each? 



3. Can we use bismuth, or some metal other than lead, in the man- 

 ufacture of transparent and colorless glass?* 



4. As the inflexion of light by angular projections, produces nearly 

 the same dispersion that refraction does, and as the best of our polish- 

 ing probably leaves the surface of glass rough and uneven, which 

 would be obvious if we could see the ultimate atoms, may not a con- 

 siderable part of the dispersion be derived from the inflexion by the 

 irregularly situated particles at the surface ? 



5. As the combination of bismuth with some other metals adds 

 much to their fluidity in the melted state, would not the oxide of bis- 

 muth probably add much to the fluidity of glass in the melted state? 



* We should presume that oxide of bismuth would give a yellow color to glass, 

 and it is quite doubtful whether we could impart any portion of the fluidity which 

 belongs to the alloys of metallic bismuth to the compounds of its oxide with alka- 

 line and earthy bases. — Eds. 



