PHILOSOPHICAL TRANSACTIONS. 



339 



VOL. XI.] 



by a less accurate measure, as I suspect by the conjectural measure of the re- 

 fraction of his prism by the ratio of the sines 2 to 3, set down at the same 

 time, instead of an experimental one, then might it be two or three degrees 

 less than 60, if not still less; and all this, if it should be so, would take away 

 the greatest part of the difference between us. 



But however it be, I am well assured my own observation was exact enough. 

 For I have repeated it divers times since the receipt of Mr. Lucas's letter, and 

 that without any considerable difference of my observations, either from each 

 other or from what I wrote before. And that it might appear experimentally, 

 how the increase of the angle increases the length of the image, and also that 

 nobody who has a mind to try the experiment exactly, might be troubled to pro- 

 cure a prism which has an angle just of the size assigned by me; I tried the 

 experiment with divers angles, and have set down my trials in the following 

 table: where the first column expresses the six angles of two prisms which I 

 used, which were measured as exactly as I could, by applying them to the angle 

 of a sector; and the second column expresses in inches the length of the image 

 made by each of those angles ; its breadth being 1 inches, its distance from the 

 prism 1 8 feet 4 inches, and the breadth of the hole in the window shutter -i- of 

 an inch. 



Lengths ^ Lengths 



of image. » ^* of image. 



74- Second prism. ^ 54° o' 7i 



9i 62 12 10^ 



10^ ^ 63 48 101- 



First prism. ^ 



Angles. 



56° 10' 

 60 24 

 63 26 



You may perceive, that the length of the images, in respect of the angles 

 that made them, are something greater in the second prism than in the first ; 

 but that was because the glass, of which the second prism was made, had the 

 greater refractive power. The days in which I made these trials were pretty 

 clear, but not so clear as I desired, and therefore afterwards meeting with a day 

 as clear as I desired, I repeated the experiment with the second prism, and foimd 

 the lengths of the image made by its several angles to be about ^ of an inch 

 greater than before, the measures being those set down in this table. 



r Angles. 



The second prism. ^ ^^ 



0' 

 62 12 

 I 63 48 



Lengths 

 of image. 



/ 3 

 104- 

 11 



