262 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 675-6. 



that imposes on Mr. Line I cannot imagine ; but I suspect he has not tried the 

 experiment since he acquainted himself with my theory, but depends upon his 

 old notions, taken up before he had any hint given to observe the figure of the 

 coloured image. I shall desire him therefore, before he returns any answer, to 

 try it once more for his satisfaction, and that according to this manner. 



Let him take any prism, and hold it so that its axis may be perpendicular to 

 the sun's rays, and in this posture let it be placed as close as may be to the hole 

 through which the sun shines into a dark room, which hole may be about the 

 size of a pea. Then let him turn the prism slowly about its axis, and he shall 

 see the colours move upon the opposite wall, first towards that place to which 

 the sun's direct light would pass, if the prism were taken away, and then back 

 again. When they are in the middle of these two contrary motions, that is, 

 when they are nearest that place to which the sun's direct ray tends, there let 

 him stop ; for then are the rays equally refracted on both sides the prism. In 

 this posture of the prism let him observe the figure of the colours, and he shall 

 find it not round as he contends, but oblong, and so much the more oblong as 

 the angle of the prism, comprehended by the refracting plains, is larger, and 

 the wall on which the colours are cast, more distant from the prism ; the colours 

 red, yellow, green, blue, purple, succeeding in order, not from one side of the 

 figure to the other, as in Mr. Line's conjecture, but from one end to the other; 

 and the length of the figure being not parallel but transverse to the axis of the 

 prism. After this manner I used to try the experiment: for I have tried it 

 often ; sometimes to observe the circumstances of it, sometimes in order to 

 further experiments, and sometimes to show it to others, and in all my trials the 

 success was the same. But whereas Mr. Line thinks, I tried it in a cloudy day, 

 and placed the prism at a great distance from the hole of the window ; the 

 experiment will not succeed well if the day be not "clear, and the prism placed 

 close to the hole, or so near at least, that all the sun's light that comes from 

 the hole may pass through the prism also, so as to appear in a round form if in- 

 tercepted by a paper immediately after it has past the prism. 



When Mr. Line has tried this, I could wish he would proceed a little further 

 to try that which I called the experimentum crucis, seeing, if I mis-remember 

 not. he denies that as well as the other. For when he has tried them, which 

 by his denying them, I know he has not done yet as they should be tried, I 

 presume he will rest satisfied. 



Three or four days after you gave me a sight of Mr. Line's second letter, I 

 remember I thereupon showed the first of these two experiments to that gentle- 

 man whom you found with me, when you gave me that visit, and whilst I was 

 showing it to him, R. H, a member of the Royal Society came in, and I showed 



