\^OL. L.] , PHILOSOPHICAL TRANSACTIONS. ^69 



to differ so very remarkably in the result, Mr. D. does not take upon him to 

 account for ; but only adds, that he used all possible precaution and care in the 

 process, and that he kept the apparatus by him to evince the truth of what he 

 wrote, whenever he might be properly required so to do. 



He plainly saw then, that if the refracting angle of the water-vessel could have 

 admitted of a sufficient increase, the divergency of the coloured rays would 

 have been greatly diminished, or entirely rectified ; and there would have been a 

 very great refraction without colour, as now he had a great discolouring without 

 refraction : but the inconveniency of so large an angle, as that of the vessel must 

 have been to bring the light to an equal divergency with that of the glass prism, 

 whose angle was about 6o degrees, made it necessary to try some experiments of 

 the same kind by smaller angles. 



He ground a wedge of common plate glass to an angle of somewhat less than 

 Q degrees, which refracted the mean rays about 5 degrees. He then made a 

 wedge-like vessel, as in the former experiment, and filling it with water, ma- 

 naged it so that it refracted equally with the glass wedge ; or in other words, the 

 difference of their refractions was nothing, and objects viewed through them ap- 

 peared neither raised nor depressed. This was done with an intent to observe the 

 same thing over again in these small angles which he had seen in the prism : and 

 it appeared indeed the same in proportion, or as near as could be judged; for 

 notwithstanding the refractions were here also equal, yet the divergency of the 

 colours by the glass was vastly greater than that by the water ; for objects seen 

 by these two refractions were very much discoloured. Now this was a demon- 

 stration, that the divergency of the light, by the different refrangibility, was far 

 from being equal in these two refractions. He also saw, from the position of 

 the colours, that the excess of divergency was in the glass ; so that he increased 

 the angle of the water-wedge by different trials, till the divergency of the light 

 by the water was equal to that by the glass ; that is, till the object, though con- 

 siderably refracted by the excess of the refraction of the water, appeared never- 

 theless quite free from any colours proceeding from the different refrangibility of 

 light ; and, as near as he could then measure, the refraction by the water was 

 about 4 of that by the glass. Indeed he was not very exact in taking the mea- 

 sures, because the business was not at that time about the proportions, so much 

 as to show that the divergency of the colours, by different substances, was by no 

 means in proportion to the refractions ; and that there was a possibility of re- 

 fraction without any divergency of the light at all. 



Having, about the beginning of the year 1757, tried these experiments, he 

 soon after set about grinding telescopic object-glasses on the new principles of 

 refractions, which he had gathered from them ; which object-glasses were com- 

 pounded of two spherical glasses with water between them. These glasses he 



