268 Dr. Herschel's Investigation of the Powers 



perfectly. The maximum of illumination lies in the brightest 

 yellow, or palest green. The green itself is nearly equally 

 bright with the yellow ; but, from the full deep green, the illu- 

 minating power decreases very sensibly. That of the blue is 

 nearly upon a par with that of the red ; the indigo has much 

 less than the blue ; and the 1 violet is very deficient. 



With regard to the principle of distinctness, there appears to 

 be no deficiency in any one of the colours. In the violet rays, 

 for instance, some of the experiments mention that I saw 

 badly; but this is to be understood only with respect to the 

 number of small objects that could be perceived ; for, although I 

 saw fewer of the points, those which remained visible were always 

 as distinct as, in so feeble an illumination, could be expected. It 

 must indeed be evident, that by removing the great obstacle to 

 distinct vision, which is, the different refrangibility of the rays 

 of light, a microscope will be capable of a much higher degree 

 of distinctness than it can be under the usual circumstances. A 

 celebrated optical writer has formerly remarked, that a fly, illu- 

 minated by red rays, appeared uncommonly distinct, and that 

 all its minute parts might be seen in great perfection ; and, from 

 the experiments which have been related, it appears that every 

 other colour is possessed of the same advantage. 



I am well aware that the results I have drawn from the fore- 

 going experiments, both with regard to the heating and illumi- 

 nating powers of differently-coloured rays, must be affected by 

 some little inaccuracies. The prism, under the circumstances 

 in which I have used it, could not effect a complete separation 

 of the colours, on account of the apparent diameter of the sun, 

 and the considerable breadth of the prism itself, through which 

 the rays were transmitted. 



