268 PHILOSOPHICAL THANSAGTIONS. [aNNO 1758. 



refractions miist produce equal divergencies in every sort of medium : whence it 

 should also follow, that equal and contrary refractions must not only destroy each 

 other, but that the divergency of the colours from one refraction would likewise 

 he corrected by the other ; and there could be no possibility of producing any 

 such thing as refraction, which would not be affected by the different refrangibi- 

 lity of light ; or, in other words, that however a ray of light might be refracted 

 backward and forward by different mediums, as water, glass, &c. provided it was 

 so done that the emergent ray should be parallel to the incident one, it would 

 ever after be white; and conversely, if it should come out inclined to the incident, 

 it would diverge, and ever after be coloured. From which it was natural to 

 infer, that all spherical object-glasses of telescopes must be equally affected by the 

 different refrangibility of light, in proportion to their apertures, whatever ma- 

 terial they may be formed of. 



But it seems worthy of consideration, that notwithstanding this notion has 

 been generally adopted as an incontestable truth, yet it does not seem to have 

 been hitherto so confirmed by evident experiment, as the nature of so important 

 a matter justly demands ; and this it was that determined Mr. D. to attempt 

 putting the thing to issue by the following experiment. He cemented together 

 two plates of parallel glass at their edges, so as to form a prismatic or wedge- 

 like vessel, when stopped at the ends or bases ; and its edge being turned down- 

 ward he placed therein a glass prism with one of its edges upward, and filled up 

 the vacancy with clear water ; thus the refraction of the prism was contrived to 

 be contrary to that of the water, so that a ray of light transmitted through both 

 these refracting mediums would be refracted by the difference only between the 

 two refractions. Therefore, as he found the water to refract more or less than 

 the glass prism, he diminished or increased the angle between the glass plates, 

 till he found the two contrary refractions to be equal ; which he discovered by 

 viewing an object through this double prism ; which, when it appeared neither 

 raised nor depressed, he was satisfied that the refractions were equal, and that 

 the emergent rays were parallel to the incident. 



Now, .according to the prevailing opinion, the object should have appeared 

 through this double prism quite of its natural colour ; for if the difference of re- 

 frangibility had been equal in the two equal refractions, they would have recti- 

 fied each other : but the experiment fully proved the fallacy of this received 

 opinion, by showing the divergency of the light by the prism to be almost 

 double of that by the water ; for the object, though not at all refracted, was 

 yet as much infected with prismatic colours, as if it had been seen through a 

 glass wedge only, whose refracting angle was near 30 degrees. This experiment 

 will be readily perceived to be the same as that which Sir Isaac Newton men- 

 tions, book 1, part 2, prop. 3, experiment 8, of his Optics. But how it comes 



