1899.] on Transparency and Opacity. 117 



the same thickness an effort was made to reduce the difference of 

 dispersive powers. To this end the powder was of plate glass and 

 the liquid oil of cedar-wood adjusted with a little bisulphide of carbon. 

 The general transparency of this cell was the highest yet observed. 

 When it was tested upon the spectrum, the range of refrangibility 

 transmitted was estimated at 34 times the interval of the D's. 



As regards the substitution of other transparent solid material 

 for glass, the choice is restricted by the presumed necessity of avoiding 

 appreciable double refraction. Common salt is singly refracting, 

 but attempts to use it were not successful. Opaque patches always 

 interfered. With the idea that these might be due to included mother 

 liquor, the salt was heated to incipient redness, but with little advan- 

 tage. Transparent rock-salt artificially broken may, however, be used 

 with good effect, but there is some difficulty in preventing the approxi- 

 mately rectangular fragments from arranging themselves too closely. 



The principle of evanescent refraction may also be applied to the 

 spectroscope. Some twenty years ago, an instrument had been con- 

 structed upon this plan. Twelve 90° prisms of Chance's " dense 

 flint " were cemented, in a row upon a strip of glass (Fig. 1), and the 

 whole was immersed in a liquid mixture of bisulphide of carbon with 

 a little benzole. The dispersive power of the liquid exceeds that of 

 the solid, and the difference amounts to about three-quarters of the 



Fig. 1. 



dispersive power of Chance's " extra dense flint." The resolving 

 power of the latter glass is measured by the number of centimetres of 

 available thickness, if we take the power required to resolve the 

 D-lines as unity. The compound spectroscope had an available 

 thickness of 12 inches or 30 cm., so that its theoretical resolving 

 power (in the yellow region of the spectrum) would be about 22. 

 With the aid of a reflector the prism could be used twice over, and 

 then the resolving power is doubled. 



One of the objections to a spectroscope depending upon bi- 

 sulphide of carbon is the sensitiveness to temperature. In the 

 ordinary arrangement of prisms the refracting edges are vertical. If, 

 as often happens, the upper part of a fluid prism is warmer than the 

 lower, the definition is ruined, one degree (Centigrade) of temperature 

 making nine times as great a difference of refraction as a passage 

 from D x to D 2 . The objection is to a great extent obviated by so 

 mounting the compound prism that the refracting edges are horizontal, 

 which of course entails a horizontal slit. The disturbance due to a 

 stratified temperature is then largely compensated by a change of focus. 



In the instrument above described the dispersive power is great — 



