VOL. VI.] PHILOSOPHICAL TRANSACTIONS. (JS/ 



now found due to something else, we liave as good reason to believe that to be 

 a substance also. 



Besides, whoever thought any quality to be a heterogeneous aggregate, such 

 as light is discovered to be. But, to determine more absolutely what light is, 

 after what manner refracted, and by what modes or actions it prorluces in our 

 minds the phantasms of colours, is not so easy. And I shall not mingle con- 

 jectures with certainties. 



Reviewing what I have written, I see the discourse itself will lead to divers 

 experiments sufficient for its examination, and therefore I shall not trouble 

 you further, than to describe one of those which I have already insinuated. 



In a darkened room make a hole in the shut of a window, whose diameter 

 may conveniently be about a third part of an inch, to admit a convenient quan- 

 tity of the sun's light; and there place a clear and colourless prism, to refract 

 the entering light towards the further part of the room, which, as I said, will 

 thereby be diffused into an oblong coloured image. Then place a lens of about 

 three feet radius (suppose a broad object glass of a three-foot telescope,) at the 

 distance of about four or five feet from thence, through which all those colours 

 may at once be transmitted, and made by its refraction to convene at a further 

 distance of about ten or twelve feet. If at that distance you intercept this light 

 with a sheet of white paper, you will see the colours converted into whiteness 

 again by being mingled. But it is requisite, that the prism and lens be placed 

 steady, and that the paper on which the colours are cast be moved to and fro; 

 for by such motion, you will not only find at what distance the whiteness is 

 most perfect, but also see how the colours gradually convene, and vanish into 

 whiteness, and afterwards having crossed one another in that place where they 

 compound whiteness, are again dissipated and severed, and in an inverted order 

 retain the same colours which they had before they entered the composition. 

 You may also see, that if any of the colours at the lens be intercepted, the 

 whiteness will be changed into the other colours. And therefore that the com- 

 position of whiteness be perfect, care must be taken that none of the colours 

 fall beside the lens. 



In the annexed design of this experiment, ABC expresses the prism set end- 

 wise to sight, fig. 14, pi. 14, close by the hole F of the window EG. Its ver- 

 tical angle ACB may conveniently be about 6o degrees: MN designs the lens. 

 Its breadth 2^- or 3 inches. SF one of the straight lines, in which difform rays 

 may be conceived to flow successively from the sun. FP and FR two of those 

 rays unequally refracted, which the lens makes to converge towards Q, and after 

 decussation to diverge again. And HI the paper, at divers distances, on which 



