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Now, according to the prevailing opinion, the object should have appeared thro' this double prism quite of its natural colour; for if the difference of refrangibility had been equal in the two equal refractions, they would have rectified each other: but the experiment fully proved the fallacy of this received opinion, by shew^ing the divergency of light by the prism to be almost double of that by the water; for the object, tho' ^not [[strikethrough]] at [[/strikethrough]] at all refracted, was yet as much infected with prismatic colours, as if it had been seen thro' a glass wedge only, phase refracting angle was near 30 degrees. 

N.B. This experiment will [[strikethrough]] really [[/strikethrough]] be readily perceived to be the same as that which Sir Isaac Newton mentions; (Book I. Part ii. Prop. 3. Experiment 8. of his optics) but how it comes to differ so very remarkably in the result, I shall not take upon me to account for; but will only add, that I used all possible precaution and care in the process, and that I kept the apparatus by me to evince the truth of what I write, whenever I may be properly required so to do. 

I plainly saw then, that if the refracting angle of the [[strikethrough]] vessel [[/strikethrough]] water–vessel could have admitted of a sufficient increase, the divergency of the coloured rays would have been greatly diminished, or intirely rectified; and there would have been a very great refraction without colour, as now I had a very great discolouring without refraction: but the inconveniency of 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 60 degrees, made it necessary to try some experiments of the same kind, by smaller angles. 

I ground a wedge of common plate glass to an angle somewhat less than 9 degrees, which refracted the mean rays about 5 degrees. I then made a wedge–like vessel, as in the former experiment, and filling it with water, managed it so, that it refracted equally with the glass wedge; or, in other words, the difference of their refractions were nothing, and objects viewed thro' them appeared neither raised nor depressed. This was done with an intent to observe the same thing over again in these small angles, which I had seen in the prism: and it appeared indeed the same in proportion, or as near as I could judge; for notwithstanding the refractions [[strikethrough]] have also [[/strikethrough]] 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 demonstration, that the divergency of the light, by the different refrangibility, was far from being equal in these two refractions. I also saw, from the position of the colours, that the excess of the divergency was in the glass; so that I 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, tho' considerably refracted, by the excess of the refraction of the water, appeared nevertheless quite free from any colours proceeding from the different refrangibility of light; and, as near as I could then measure, the refraction by the water was about 5/4 of that by the glass. Indeed I was not very exact in taking the measures, because my business was not at that time about the proportions, so much as to shew, that the divergency of the colours, by different substances, was by no means in proportion to
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