158 DISPERSION OF LIGHT. SECT. XIX. 



were supposed to be absorbed by the atmosphere of the 

 sun. If they were absorbed by the earth's atmosphere, 

 the very same rays would be wanting in the spectra 

 from the light of the fixed stars, which is not the case ; 

 for it has already been stated that the position of the 

 dark lines is not the same in spectra from starlight and 

 from the light of the sun. The solar rays reflected 

 from the moon and .planets would most likely be mod- 

 ified also by their atmospheres, but they are not : for 

 the dark lines have precisely the same positions in the 

 spectra, from the direct and reflected light of the sun. 

 But the annular eclipse which happened on the 15th of 

 May, 1836, afforded Professor Forbes the means of 

 proving that the dark lines in question cannot be attrib- 

 uted to the absorption of the solar atmosphere ; they 

 were neither broader nor more numerous in the spec- 

 trum formed during that phenomenon than at any other 

 time, though the rays came only from the circumference 

 of the sun's disc, and consequently had to traverse a 

 greater depth of his atmosphere. We are therefore 

 still ignorant of the cause of these rayless bands. 



A sunbeam received on a screen, after passing through 

 a small round hole in a window-shutter, appears like a 

 round white spot ; but when a prism is interposed, the 

 beam no longer occupies the same space. It is separa- 

 ted into, the prismatic colors, and spread over a line of 

 considerable length, while its breadth remains the same 

 with that of the white spot. The act of spreading or 

 separation is called the dispersion of the colored rays. 

 Dispersion always takes place in the plane of refraction, 

 and is greater as the angle of incidence is greater. It 

 varies inversely as the length of a wave of light, and 

 directly as its velocity : hence toward the blue end of 

 the spectrum, where the undulations of the rays are 

 least, the dispersion is greatest. Substances have veiy 

 different dispersive powers ; that is to say the spectra 

 formed by two equal prisms of different substances under 

 precisely the same circumstances, are of different 

 lengths. Thus, if a prism of flint glass and one of crown 

 glass of equal refracting angles be presented to two rays 

 of white light at equal angles, it will be found, that the 

 space over which the colored rays are dispersed by the 



