CHAPTEE III. 



DISPERSION. 



(45) WE have hitherto supposed light to be simple or ho- 

 mogeneous. The light of the Sun, however, and most of the 

 lights, natural or artificial, with which we are acquainted, are 

 compound, each ray consisting of an infinite number of rays 

 differing in colour and ref Tangibility . This important dis- 

 covery we owe to Newton. We shall briefly describe the 

 principal experiments by which its truth was established. 



(46) When a beam of solar light is admitted into a dark- 

 ened room through a small circular aperture, and received on 

 a screen at a distance, a circular image of the Sun will be de- 

 picted there, whose diameter will correspond to that of the hole. 

 If now the light be intercepted by a prism, having its refracting 

 edge horizontal and perpendicular to the incident beam, the 

 image of the Sun will be thrown upwards by the refraction 

 of the prism, and will be no longer white and circular, but 

 coloured and oblong ; the sides which are perpendicular to the 

 edge of the prism being rectilinear and parallel, and the ex- 

 tremities semicircular. The breadth of this image, or spectrum 

 (as it is called), is equal to the diameter of the unrefracted 

 image of the Sun, but its length is much greater. 



Now if the solar beam consisted of rays having all the same 

 refrangibility, the refracted image should be circular, and of 

 the same dimensions as the unrefracted image, from which it 

 should differ only in position. For the rays composing the 

 beam, being parallel at their incidence on the prism, must 

 (on this supposition) be equally refracted by it, and there- 



