SECT, xix.] DISPERSION OF LIGHT. 183 



rays reflected from the moon and planets would most likely 

 be modified 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 attributed to the absorp- 

 tion of the solar atmosphere ; they were neither broader nor 

 more numerous in the spectrum formed during that pheno- 

 menon 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 separated into the 

 prismatic colours, 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 coloured 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 towards 

 the blue end of the spectrum, where the undulations of the 

 rays are least, the dispersion is greatest. Substances have 

 very 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 coloured rays are dispersed by the flint glass is 

 much greater than the space occupied by that produced by 

 the crown glass : and, as the quantity of dispersion depends 

 upon the refracting angle of the prism, the angles of the 



