optics. i2i7 



Of the different Refrangibility of the Rays of 

 Light. 



Hitherto, in treating of the refraction of light, 

 we supposed that all the particles of light, in pass- 

 ing out of one medium into another of different 

 density, are equally refracted. This was the opi- 

 nion of philosophers before the time of Sir Isaac 

 Newton ; but that indefatigable investigator disco- 

 vered that light is not a simple homogeneous body, 

 but is compounded of different species ; and that 

 each species is disposed both to suffer a different 

 degree of refrangibility, in passing out of one me- 

 dium into another, and to excite in our mind the 

 idea of a different colour from the rest ; and that 

 bodies appear of that colour which they are most 

 disposed to reflect. 



To show this, let a room be darkened, and let a 

 beam of light, B, (Plate 26. fig. 1.) be permitted to 

 shine into it through a small hole in the window- 

 shutter, and be made to fall upon a glass prism, A ; 

 then will the sun's rays, in passing through it, 

 suffer different degrees of refraction, and by that 

 means be parted into different rays, which being 

 received upon a sheet of white paper will exhibit 

 the following colours ; viz. red, orange, yellow, 

 green, blue, indigo, and violet ; and if the whole 

 spectrum, or image, C D, be divided into 360 equal 

 parts, the red will occupy 45 of them, the orange 

 27, the yellow 48, the green 60, the blue 60, the 

 indigo 40, and the violet 80. 



As a ray of the sun may be separated into these 

 seven primitive colours, so, by their mixture again, 

 may white be produced. 



it 4 



