Composition of White Light. fi3 



it is equally demonstrable, that with heterogeneous light, 

 gradually and imperceptibly differing in refrangibility, the 

 images of such bodies, formed in the same manner, must 

 necessarily be elongated : the blending together of innu- 

 merable circles, constantly separating, but which no dis- 

 tance, within the limits of our experiments can separate, 

 must produce images of this description. 



If, then, we can produce by means of the prism, circular 

 images of circular objects, as in the experiments of Newton, 

 whether in one or more colours, we may conclude, that the 

 light by which these images are formed, taking each image 

 separately, is homogeneous. 



The observations which I have here introduced, and va- 

 rious other considerations, long since convinced me that, 

 there are only as many degrees of refrangibility as there 

 are colours ; and these I have endeavoured to show, are 

 limited to three : but in making experiments upon the 

 absorbtion of the light of the sun, refracted by a prism, by 

 means of coloured glasses, I discovered images of the lumi- 

 nous body, which could not be thus accounted for ; and, 

 from their general appearance, it occurred to me, that 

 these images were formed by rays of light which had under- 

 gone various reflexions within the prism. 



In pursuing these and other experiments, particularly 

 those mentioned at the close of my last paper, I discovered 

 that the absorbtion of light by a coloured medium, when 

 the quantities were varied in different experiments, was not 

 in proportion to the quantity incident upon its surface, but 

 that it bore a higher ratio in weak, than in strong light :* 

 at least, what is sufficient for my present purpose, weak 

 light is rendered invisible by an absorbing medium, which, 

 under circumstances in other respects similar, transmits 

 stronger light in quantities sufficient to form distinct images. 

 Upon making this discovery, which in itself, perhaps, 

 may be of little consequence, it struck me that the images 

 before mentioned, if formed of reflected light, would be 

 wholly absorbed by coloured media, while so much of the 

 direct light might be transmitted, as would give correct 

 images of the luminous body in the primitive colours. 



A great variety of experiments fully confirmed these 

 views; and I have now no great difficulty, with a good 

 * This must !«■ considered merer* ;i bypotherisi 



