VOL. XXIX.] PHILOSOPHICAL TRANSACTIONS. 231 



may be tried with success ; but the trial will be less troublesome, if it be made 

 in such a manner as is described in the 4th prop, of the first book, of Sir Isaac 

 Newton's Optics. 



Sir Isaac Newton therefore, on reading what has been cited out of the Acta 

 Eruditorum, desired Mr. Desaguliers to try the experiment in the manner 

 described in the said proposition ; and he tried it accordingly with success before 

 several gentlemen of the Royal Society, and afterwards before M. Monmort 

 and others of the Royal Academy of Sciences ; and still shows it to those who 

 desire to see it. How this and other concomitant experiments were tried and 

 succeeded, is described as follows. 



Keeper. I. — Having sewed together endwise two pieces of ribbon, each 4 

 inches long, the one blue and the other red, whose common breadth was 4 of 

 an inch ; I caused it to be held in such manner, that the light which fell from 

 the clouds through the window was so reflected, that the angle made by the 

 rays of light, which came in at the middle of the window, with the plane of 

 the ribbon produced, was equal to the angle made by a line drawn from the 

 ribbon to my eye and the said plane of the ribbon. My eye was placed as far 

 behind the ribbon as the windov^r was before it, the distance from which to me 

 was about 12 feet. Then looking through a prism at the ribbon, it appeared 

 broken asunder in the place where the blue and red half joined. When the 

 prism was held with the refracting angle downwards, or laid with one of its 

 planes fiat upon the nose, the blue half of the ribbon appeared to be carried 

 down lower than the red, as at br, in fig. 1, pi. 6; but when the refracting 

 angle of the prism was turned upwards, as when the prism has one of its 

 planes laid flat to the forehead, then the blue half of the ribbon was lifted up, 

 as at €^. 



The prism was of white glass, having every angle of 60 degrees ; but when 

 instead of it, one of a greenish sort of glass, such as object glasses of telescopes 

 are made of, was used, having the refracting angle which I looked through of 

 about 48 degrees; the same phenomenon was more distinct, this glass having 

 no veins, but the red and blue were nearer to a straight line : in such manner, 

 that if A represent the ribbon seen through the first prism, b will represent the 

 ribbon seen through the second prism, fig. 2. U the refracting angle of the 

 last prism had been as great as that of the first, the light being transmitted 

 through too great a body of greenish glass, the phenomenon would not have 

 succeeded so well. 



The blue ribbon being somewhat too pale, and the red a little dull, I repeated 

 the experiment with a screen of blue, and one of red worsted, joined together 

 in the middle, as the ribbons were before; and, the colours of both being 



