of the prismatic Colours to heat and illuminate Objects. 26*0. 



Perhaps an arrangement like that in Fig. 16, of the New- 

 tonian experiments, might be employed ; if instruments of suf- 

 ficient sensibility, such as air thermometers, can be procured, 

 that may be affected by the enfeebled illumination of rays that 

 have undergone four transmissions, and eight 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 refinements, 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 likewise evi- 

 dent, that at least the red light of the prismatic spectrum, was 

 much less adulterated than any of the other colours ; their re- 

 fractions 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 foun- 

 dation. 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 removed ; 

 therefore, on account of the coloured points, which have been 

 mentioned in the 6th and 7th experiments, I was willing to try 



