212 



SENSE OF SIGHT. 



Violet 



Indigo 



Blue 



Green 



Yellow 



Orange 



Red 



White 



the direction g G, the beam will expand ; and if, after having emerged, 



it be received on the 

 whitened screen, M 

 N, it will be found to 

 occupy a considera- 

 ble space ; and, in- 

 stead of the white 

 spot, there will be an 

 oblong image of the 

 sun, K L, consisting 

 of seven colours ; 

 red, orange, yellow, 

 green, blue, indigo, 



;'.''' and violet. Each of 



these colours admits 

 of no farther decom- 



Prismatic Spectrum. position when again 



passed through the 



prism ; and the whole lengthened image of the sun is called the pris- 

 matic or solar spectrum. In this dispersion of the coloured rays, it 

 will be observed that the red ray is the least turned from its course ; 

 and is hence said to be the least refrangible ; whilst the violet is most so. 

 Such is the spectrum, as depicted by Newton: since his time, it has, by 

 some, been reduced to three colours, red, yellow, and blue; as certain 

 of the colours can be composed from others, the green, for example, from 

 the blue and yellow. Wollaston made it to consist of four; red, green, 

 blue, and violet; Sir J. Herschel of four; red, yellow, blue, and violet: 

 and, more recently, Sir David Brewster has restricted it to three; red, 

 yellow, and blue. The causes which have led to these various divisions, 

 it is not our province to explain. 



Each of the rays, of which the spectrum is composed, appears to 

 have a different calorific and chemical action ; but this also is a subject, 

 that nowise concerns the function under consideration. 



The decomposition of light into its constituent rays enables us to 

 explain the cause of the colour of different substances. When white 

 light impinges upon a body, the body either absorbs all the rays that 

 compose it ; reflects all ; or absorbs some, and reflects others. If it 

 reflects the whole of the light to the eye, it is of a white colour; if it 

 absorbs all, or reflects none, it is black; if it reflects only the red ray, 

 and absorbs all the rest, it is red; and so of the other colours. The 

 cause, why one body reflects one ray, or set of rays, and absorbs 

 others, is unknown. It is conceived to be owing to the nature and 

 particular arrangement of its molecules; which is probable. But we 

 are still as much in the dark as ever. It is accounting for the ignotum 

 per ignotius. 



Two other points require a brief notice, being intimately concerned 

 in vision ; the aberration of sphericity, and aberration of refrangibility. 

 It has been remarked, that rays of light after passing through a con- 

 vex lens, or medium whose surfaces are convex converge, and are 

 brought to a focus behind it. The whole of the rays do not, however, 



