ASTRO-PHYSICS 267 



hours or days after the exposure. In several 

 observatories, systematic records are kept of 

 the state of the sky from night to night, and, 

 more than once, when a new star has been 

 detected, its previous history has been unfolded 

 by reference to photographic plates exposed 

 before the existence of the new star was sus- 

 pected. 



Two methods of obtaining spectra are known 

 to the physicist, the instruments used being 

 respectively the prism and the grating. The 

 grating consist of a number of equidistant 

 parallel scratches ruled on a reflecting surface 

 of polished metal or on a transparent surface of 

 glass. The scratches are very close together, 

 many thousands of them being included in the 

 space of an inch. When a wave of light falls 

 on a metallic grating, the scratches refuse to 

 reflect the light. The distances between the 

 scratches are comparable with the minute wave- 

 lengths of light, and thus different waves are 

 differently treated by the grating. The com- 

 ponent rays of a complex beam of light are 

 separated from each other, and, if the source 

 of light be a narrow slit, a number of parallel 

 images are formed, and a spectrum is obtained. 

 The deviation of any particular wave, such as 

 the yellow sodium ray, will depend on the wave- 

 length of the light, and, for the same grating, 

 will depend on this wave-length alone. The 

 spectral lines obtained will therefore have posi- 

 tions simply depending on the wave-length or 

 * period of vibration of the corresponding rays 

 of light ; in this differing from the similar lines 

 given by the prism, which depend in position 



