6i6 THE POPULAR SCIENCE MONTHLY. 



side of tlie mass, so that its defects may be located and the perfect 

 portions utilized to the best advantage. The mass is cut into 

 slabs suitable for working up into prisms, lenses, and other opti- 

 cal instruments. 



When a large disk is to be made, such as the great lens of a 

 refracting astronomical telescope, several attempts are frequently 

 necessary before success is gained. Two or even three years may 

 pass before suitable material is cast. 



As the result of this very troublesome process, we have slabs 

 of fairly homogeneous glass from 3'4 to 3'G times as heavy as 

 water. Although the greater density is about equal to that of 

 the diamond, the refractive indices of the two substances are 

 not the same. That of the diamond is 2'5 and of the flint glass 

 1"61. But even this refractive power is the key to many mys- 

 teries. The trouble of producing the material counts as nothing 

 in face of the results. 



The possibility of increasing the refractive index of optical 

 glass by increasing the density early attracted the attention of 

 experimenters. Since the beginning of the century attempts have 

 been made in this direction. By the use of the heavier rare 

 metals, such as thallium, a glass has been produced over five times 

 as heavy as water. The material has served admirably for the 

 manufacture of artificial jewels, but has not as yet found perma- 

 nent application in science. 



The refractive power of glass, by which the rays of light are 

 bent out of their course and images of objects formed, has its dis- 

 advantages as well as its merits. It is almost impossible to con- 

 struct a lens which shall converge the rays of light without, at 

 the same time, producing rainbow colors around the image. This 

 defect is called chromatic aberration, and, as one can readily see, is 

 fatal to the definition of the lens. It is commonly overcome by 

 employing a compound lens, made of flint and crown glass. The 

 different refractive indices of the materials correct each other's 

 aberration and produce white light. A lens so constructed is 

 termed achromatic, since it does away with the fringe of color. 

 Loss of power is naturally the price of such a correction. These 

 difficulties led to the project of making lenses out of a material 

 which should obviate the color fringe by something in the glass 

 itself. It is found that titanic and boric acids have a marked 

 effect upon the refraction of the differently colored rays, and 

 compounds of these materials have been used to good purpose. 

 We have here a field well worth further exploration. 



The best flint glass for optical use is made in Europe. It is 

 an interesting circumstance that the great establishments of 

 Messrs. Chance and Company, at Birmingham, and M. Feil et 

 Cie. now M. Mantois at Paris, which largely supply the Ameri- 



