DISPERSION. 41 



the hole, equal to it in magnitude. A prism being then placed 

 immediately behind the lens, this image will be dilated in 

 length, its breadth remaining unaltered, and thus a spectrum 

 will be formed whose breadth is the diameter of the hole ; 

 whereas, without this contrivance, the breadth would be equal 

 to that diameter, together with a line which (at the distance 

 of the screen. from the hole) subtends an angle equal to the 

 apparent diameter of the Sun. Thus, by diminishing the 

 diameter of the aperture, the breadth of the spectrum, and 

 therefore the mixture, may be reduced at pleasure. 



If the diameter of the aperture be very small, the spec- 

 trum is reduced to a narrow line, and is unfit for examination. 

 To remedy this, Newton employed a narrow rectangular 

 aperture, whose length, parallel to the edge of the prism, may 

 be as great as we please, while its breadth is very small. In 

 this manner we obtain a spectrum as broad as we wish, and 

 whose light is as simple as before. 



(50) In order to determine the laws of dispersion, it is 

 necessary to find experimentally the indices of refraction of 

 the several species of simple light, of which solar light is com- 

 posed. 



Newton's method was to determine the refractive indices of 

 the extreme red and violet rays directly by means of the formula 

 of Article (32), and to deduce those of the other rays by a simple 

 proportion. When the refracting prism was of crown-glass, 

 the indices of refraction of the extreme rays were found to be 



*77 *7& 



and ^7-, respectively. To determine the refractive indices 

 oU oU 



of the intermediate rays, it was necessary to measure the 

 spaces which they occupied in the spectrum. For this pur- 

 pose Newton delineated on paper the spectrum AHAa, and 

 distinguished it by the cross lines A0, B6, Cc, &c., drawn at 

 the confines of the several colours ; so that the space 



