268 PHYSICAL SCIENCE 



on the qualities of the glass as well as on the 

 periodic times of vibration of the different rays. 



The sharpness of definition of a spectrum 

 taken from a grating depends on the accuracy 

 with which the scratches are ruled, and thus 

 the perfection of the grating depends on our 

 power of moving the scratching tool through 

 exactly equal intervals between two scratches. 

 To control the movement a perfect screw is 

 required, and to Professor Rowland's improve- 

 ment in the manufacture of screws in 1882, and 

 to his idea of using them to rule gratings on 

 concave metallic surfaces, is directly due the 

 possibility of making adequate use of the 

 resources of photography in the province of 

 solar and stellar spectrum analysis. The arts 

 and the sciences are closely related ; an advance 

 in one of them often leads to a corresponding 

 advance in the other, and it is not always 

 science that leads the way. 



The concave grating banished the need for a 

 lens to focus the rays after diffraction, and an 

 image of the spectrum could now be obtained 

 from the grating alone. Glass is opaque to much 

 of the ultra-violet radiation, in which sunlight, at 

 any rate, is very rich. Prismatic spectra and 

 spectra taken with plane gratings and lenses do 

 not show the ultra-violet lines. But, by the use 

 of a concave grating and a reflecting telescope, 

 the presence of glass becomes unnecessary, and 

 investigation can be prolonged into the ultra- 

 violet region till the increasing absorption of 

 the earth's atmosphere for waves of shorter 

 and shorter wave-length prevents the rays from 

 reaching the surface of the ground. 



