144 MODERN SCIENCE READER 



somewhat similar in their optical properties. That is to 

 say, while their refractive index and dispersive power 

 varied within certain limits, they ran parallel throughout, 

 so that if the known glasses were arranged in order of 

 increasing refractive index, this would at the same time 

 place them in the order of their dispersive powers. Glasses 

 were needed which should not conform to this order, and 

 it was found that especially barium and zinc possessed in 

 a high degree the property of imparting to glass a high 

 refractive index, accompanied by a comparatively low dis- 

 persive power. 



Another problem was to prepare crown and flint glass 

 which would give as nearly as possible similarly propor- 

 tioned spectra. The flint glasses then known gave a spec- 

 trum which was much more drawn out in the blue than 

 that produced by crown glass. The remedy was found in 

 the addition of boric oxide, which foreshortens the blue end 

 of the flint-glass spectrum. 



Another class of optical glasses are the colored glasses. 

 The principal problem in the preparation of these is to 

 produce as nearly as possible ideal color niters. Such a 

 glass must absorb as completely as possible some particular 

 portion of the spectrum, while transmitting the remainder. 

 The materials added to give the requisite color are the 

 oxides, sulphides and selenides of certain metals, or in cer- 

 tain cases the metal itself in a highly divided state. The 

 color imparted by such additions depends in part on the 

 composition of the glass in which they are dissolved. 



A problem of the same character is the preparation of 

 glasses which transmit very short wave lengths of light. 

 It is well known that quartz and boric acid have the prop- 

 erty of being transparent to ultra-violet rays. The pres- 

 ence of metallic oxides more or less completely destroys 

 this property, the extent to which this takes place depend- 

 ing on the nature of the metal, thus sodium has a stronger 

 effect than potassium, while lead glasses are particularly 

 opaque to short wave lengths. However, by carefully 



