50 The Outline of Science 



positions of stars with much greater accuracy than is possible 

 to the more general and flexible mounting of equatorials. The 

 recording of transit is comparatively dry work; the spectacu- 

 lar dement is entirely absent; stars are treated merely as 

 mathematical points. But these observations furnish the very 

 basis of modern mathematical astronomy, and without them 

 such publications as the Nautical Almanac and the Connaissance 

 (Jn Temps would be robbed of the greater part of their impor- 

 tance. 



2 



The Spectroscope 



We have already learnt something of the principles of the 

 spectroscope, the instrument which, by making it possible to learn 

 the actual constitution of the stars, has added a vast new domain 

 to astronomy. In the simplest form of this instrument the ana- 

 lysing portion consists of a single prism. Unless the prism is very 

 large, however, only a small degree of dispersion is obtained. It 

 is obviously desirable, for accurate analytical work, that the dis- 

 persion that is, the separation of the different parts of the 

 spectrum should be as great as possible. The dispersion can 

 be increased by using a large number of prisms, the light emerg- 

 ing from the first prism, entering the second, and so on. In this 

 way each prism produces its own dispersive effect and, when a 

 number of prisms are employed, the final dispersion is consider- 

 able. A considerable amount of light is absorbed in this way, 

 however, so that unless our primary source of light is very 

 strong, the final spectrum will be very feeble and hard to deci- 

 pher. 



Another way of obtaining considerable dispersion is by using 

 a diffraction r/ratinc/ instead of a prism. This consists essentially 

 of a piece of glass on which lines are ruled by a diamond point. 

 When the lines are sufficiently close together they split up light 

 falling on them into its constituents and produce a spectrum. 



