6 1 4 THE POPULAR SCIENCE MONTHLY. 



struments. In general, the greater the index of refraction, the 

 more available the glass. The practical question with him is to 

 know the conditions which affect the index of refraction. To 

 answer this intelligently, one must consider why the light is bent 

 at all in changing its medium. If a bather run down a smooth, 

 hard beach into the water, he is very apt to fall head foremost 

 when he reaches the denser medium. His feet are suddenly re- 

 tarded, while his body keeps on through the air with the old 

 velocity. The result is a change of direction in his motion, which 

 is in one sense disastrous. 



Precisely the same thing happens to the light. It is gen- 

 erally conceded to be a progressive wave-motion. When the 

 beam passes into a denser medium at an angle, the side of the 

 beam which enters first is retarded, while the other side keeps on 

 at the old velocity. The result is, that the whole beam is swung 

 out of line and takes a new direction in the new medium. The 

 index of refraction is simply a quantitative expression for this 

 bending, and depends upon the nature of the substance and its 

 density. The great brilliancy of the diamond is due to its very 

 high refractive index, and the sparkle of cut glass is the result 

 of a similar property. 



Since the employment of glass in optics depends upon its 

 ability to bend the rays of light to a common point or focus, its 

 value increases with its refractive power that is to say, with its 

 density. The problem set before the maker of optical glass is, 

 therefore, quite different from that which must be solved by the 

 manufacturer of more every-day goods. He must produce a 

 glass which has great weight without any loss of transparency. 

 The difficulty lies in this, that the substances which add weight to 

 the glass are prejudicial to its transparency. Success is found in 

 the nice balance between these opposing tendencies. 



Glass is a double silicate. If it is to have large density, the 

 metallic bases combined with the silica must be heavy. Hence, 

 the ordinary glass of commerce a double silicate of lime and 

 soda will not serve in optics. In place of this, a double silicate 

 of lead and potash must be used. The lead gives density to the 

 glass, and consequently high refractive power. The crude mate- 

 rials must be as pure as practicable. To about a hundred parts 

 of sand there is added a mixture of one hundred parts of minium, 

 or red lead, and thirty parts of potash. When these are fused 

 together in large, hooded crucible pots, a very liquid glass results. 

 It is considerably more fusible than the lime-soda glass. So far, 

 the process is easy ; but the silicate of lead is so much heavier 

 than the silicate of potash that when the fused mass is allowed 

 to cool the denser silicate has a decided tendency to separate out 

 at the bottom of the crucible. This makes the glass streaky and 



