TOL. VII.] PHILOSOPHICAL TRANSACTIONS. 741 



pendent; since some of the parts may be intercepted without the others, and 

 be separately reflected or refracted towards difll'rent sides. This being premi"- 

 sed, the whole force of the objection will lie in this, that colours may be length- 

 ened out by some certain diffusion of light beyond the hole, which does notarise 

 from the unequal refraction of the different rays, or of the independent parts 

 of light. And that the image is no otherwise lengthened, was shown in my 

 letter in Numb. 80, of the Transactions ; and to confirm the whole in the strictest 

 manner, I added that experiment now known by the name Experimentum Cru- 

 cis ; of the conditions of which since the Rev. Father has some doubt, I have 

 thought fit to represent it by a scheme. Let BC (fig. 10, pi. 15) then be the 

 anterior board, to which the prism A is immediately prefixed, and let DE be 

 the other board, at the distance of about 12 feet from the former, to which the 

 other prism F is affixed. And let the boards be perforated at x and y in such 

 a manner, that a little of the light refracted by the former prism may pass 

 through both the holes to the second prism, and be there refracted again. Now 

 let the former prism be turned about its axis with a reciprocal motion ; then the 

 colours falling on the latter board DE will be raised and depressed by turns : and 

 thus the several colours may at pleasure be made to pass successively through the 

 hole y to the latter prism, while all the other colours fall on the board. Then 

 you will see that the said rays of different colours will be differently refracted at 

 the latter prism, as they will be seen on different places of the opposite wall, or 

 of any obstacle GH, at the distance of some feet from it; as suppose the violet 

 rays at H, the red at G, and the intermediate rays at the intermediate places : 

 and yet, because of the determinate position of the holes, the incidence of the 

 rays of each colour through both must be similar. And thus it appears, by 

 measuring, that the rays of different colours have different laws of refractions. 



But I suspect what it was that caused the Rev. Father to doubt ; viz. it seems 

 he placed his first prism A behind the board B C, and thus by turning it about 

 its axis, it is probable that the inclination of the rays intercepted between the 

 two holes may have suffered some change by the intermediate refraction. But 

 by the description before given in the Transactions, the first board ought to be 

 placed after the prism, that the rays may pass in a straight direction between 

 the holes, agreeably to my words ; " I took two boards, and placed one of them 

 close behind the prism at the window." And the design of the experiment re- 

 quires the same thing. 



It may be further observed, that in this experiment, because of the refraction of 

 the second prism, the coloured light is much less diffused and less divergent, than 

 when it is quite white, so that the image at G or H is nearly circular; espe- 

 cially if the prisms be placed parallel^ and their angles in a contrary position, as 



