yoL. Xr.] PHILOSOPHICAL TRANSACTIONS. S41 



One thing more I shall add: that the utmost length of the image, from the 

 faintest red at one end to the faintest blue at the other, must be measured. For 

 in my first letter about colours, where I set down the length to be five times the 

 breadth, I called that length the utmost length of the image; and I measured 

 the utmost length, because I account all that length to be caused by the imme- 

 diate light of the sun, seeing the colours, as 1 noted above, become visible to 

 the greatest length in the clearest days, that is, when the light of the sun trans- 

 cends most the light of the clouds. Sometimes there will happen to shoot out 

 from both ends of the image a glaring light a good way beyond these colours, 

 but this is not to be regarded, as not appertaining to the image. If the mea- 

 sures be taken right, the whole length will exceed the length of the straight 

 sides by about the breadth of the image. 



By these things set down thus circumstantially, I presume Mr. Lucas will be 

 enabled to accord his trials of the experiment with mine; so nearly at least, 

 that there shall not remain any very considerable difference between us. For, 

 if some little difference should still remain, that need not trouble us any further, 

 seeing there may be many various circumstances which may conduce to it; such 

 as are not only the different figures of prisms, but also the different refractive 

 power of glasses, the different diameters of the sun at divers times of the year, 

 and the little errors that may happen in measuring lines and angles, or in 

 placing the prism at the window ; though, for my part, I took care to do these 

 things as exactly as I could. However, Mr. Lucas may make sure to find the 

 image as long or longer than I have set down, if he take a prism whose sides 

 are not hollow ground, but plain, or, which is better, a very little convex, and 

 whose refracting angle is as much greater than that I used, as that he has hi- 

 therto tried it with, is less; that is, whose angle is about 66 or 67 degrees, or, 

 if he will, a little greater. 



Concerning Mr. Lucas's other experiments, I am much obliged to him that 

 he would take these things so far into consideration, and be at so much pains 

 for examining them; and I thank him so much the more, because he is the 

 first that has sent me an experimental examination of them. By this I may 

 presume he really desires to know what truth there is in these matters. But 

 yet it will conduce to his more speedy and full satisfaction, if he a little change 

 the method which he has propounded, and instead of a multitude of things, try 

 only the experimentum crucis. For it is not number of experiments but weight 

 to be regarded; and where one will do, what need many? Had I thought 

 more requisite, I could have added more; for before I wrote my first letter to 

 you about colours, I had taken much pains in trying experiments about them, 

 and written a tractate on that subject, wherein I had set down at large the 



