1835.] Composition of White Light. 357 



If the light be admitted through a small aperture to the first 

 prism, it is seen through the second prism in the position 

 which it would have occupied, and in every other respect 

 the same as if the light had passed directly from the aperture 

 to the screen, without the intervention of the two prisms. 



The effect, therefore, of the interposition of a prism when 

 the light is suffered to fall directly upon the eye, is precisely 

 the reverse of what it is when the light falls upon a screen ; 

 for the violet light, which is the most elevated in the latter 

 case, is the most depressed in the former ; and, consequently, 

 the light, or the aperture by which it is admitted, being 

 viewed through a prism placed at the same distance from it 

 as the screen, will present a spectrum depressed instead of 

 elevated, and with the colours reversed : we see the light 

 in the direction in which it appears to come, which, of 

 course, is the reverse of that in which it is going. 



If we increase the size of the aperture by which the white 

 light is admitted, the distance of the prism we view it with 

 must be increased in the same proportion, in order to pro- 

 duce the complete developement of its different colours ; 

 for each colour is seen under a certain angle of deviation 

 from the proper position of the aperture ; and it is the dif- 

 ference of these angles, which is constant with the same 

 prism, that causes the developement, or separation, of the 

 different images : the fringes, therefore, increase in breadth 

 by increasing the distance of the prism, until they are equal 

 to the breadth of the aperture ; when the images of it, and, 

 consequently, the colours, are seen completely developed. 



Any objects which reflect white light may be substituted 

 for the apertures we have been speaking of; narrow strips 

 of white polished metal, when viewed with a prism, form 

 very distinct spectra : and when the metal does not exceed 

 the tenth of an inch in breadth, the three images of it may 

 be seen at a distance, which one hand can conveniently reach, 

 while the other supports the prism. The interposition of 

 the yellow glass reduces the colours of this spectrum to 

 green and red. 



There can be no question with regard to spectra thus dis- 

 tinctly formed ; but when the colours of two or more of 

 these spectra run into each other, previous to their complete 

 developement, it requires some attention to form clear con- 

 ceptions of the different combinations produced by it. 



