VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 681 



the number of small objects that could be perceived; for though 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, illuminated by red rays, appeared uncommonly distinct, and 

 that all its minute parts might be seen in great perfection; and, from the experi- 

 ments 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 foregoing experiments, 

 both with regard to the heating and illuminating powers of differently-coloureJ 

 rays, must be affected by some little inaccuracies. The prism, under the circum- 

 stances 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. Perhaps an 

 arrangement like that in fig. 16, of the Newtonian experiments, might be em- 

 ployed; if instruments of sufficient sensibility, such as air thermometers, can be 

 procured, that may be affected by the enfeebled illumination of rays that have 

 undergone 4 transmissions, and 8 refractions; and especially when their incipient 

 quantity has been so greatly reduced, in their limited passage through a small hole 

 at the first incidence. 



But it appeared most expedient for me, at present, to neglect all further refine- 

 ments, which may be attempted hereafter at leisure. It may even be presumed 

 that, had there not been some small admixture of the red rays in the other colours, 

 the result would have been still more decisive, with regard to the power of heating 

 vested in the red rays. And it is also evident, that at least the red light of the 

 prismatic spectrum, was much less adulterated than any of the other colours; their 

 refractions tending all to throw them from the red. That the same rays which 

 occasion the greatest heat, have not the power of illumination in any strong degree, 

 stands on as good a foundation. For since here also they have undergone the 

 fairest trial, as being most free from other colours, it is equally proved that they 

 illuminate objects but imperfectly. There is some probability that a ray, purified 

 in the Newtonian manner above quoted, especially in a well darkened room, may 

 remain bright enough to serve the purpose of microscopic illumination, in which 

 case more precision can easily be obtained. The greatest cause for a mixture of 

 colours however, which is, the breadth of the prism, I saw might easily be re- 

 moved; therefore, on account of the coloured points, which have been mentioned 

 in the 6th and 7 th experiments, I was willing to try whether they proceeded from 

 this mixture; and therefore covered the prism in front with a piece of pasteboard, 

 having a slit in it of about T l T of an inch broad. 



Exper. 10. The nail, at 9 feet 2 inches from the prism, r. I fix my attention 



vol. xviii. 4 S 



