264 PHILOSOPHICAL TRANSACTIONS. [aNNO 1675-6. 



but by Its different motion too : for whilst the prism is turned continually the 

 same way about its axis, the second and third move swiftly, and go always on 

 the same way till they disappear ; but the first moves slow, and grows con- 

 tinually slower till it be stationary, and then turns back again, and goes back 

 faster and faster, till it vanish in the place where it began to appear. 



If, without darkening their room, they hold the prism at their window in the 

 sun's open light, in such a posture that its axis be perpendicular to the sun 

 beams, and then turn it about its axis, they cannot miss of seeing the first 

 image ; which having found, they may double up a paper once or twice, and 

 make a round hole in the middle of it, about -|- or f of an inch broad, and hold 

 the paper immediately before the prism, that the sun may shine on the prism 

 through that hole ; and the prism being stayed, and held steady in that posture 

 which makes the image stationary; if the image then fall directly on an opposite 

 wall, or on a sheet of paper placed at the wall, suppose 15 or 20 feet from the 

 prism, or further off; they will see the image in such an oblong figure as I have 

 described, with the red at one end, the violet at the other, and a bluish green 

 in the middle : and if they obscure their room as much as they can, by drawing 

 curtains or otherwise, it will make the colours the more conspicuous. 



This direction J have set down, that nobody into whose hands a prism shall 

 fall, may find diifi.culty or trouble in trying it. But when Mr. Linus's 

 friends have tried it thus, they may. proceed to repeal it in a dark room with a 

 less hole made in their window shutter. And then I shall desire that they will 

 send you a full and clear description, how they tried it, expressing the length, 

 breadth and angles of the prism ; its position to the incident rays, and to the 

 window shutter; the largeness of the hole in "the window shutter, through 

 which the sun shined on the prism ; what side of the prism the sun shined on ; 

 and at what side the light came out of it again ; the distance of the prism from 

 the opposite paper or wall on which the refracted light was cast perpendicularly ; 

 and the length, breadth, and figure of the space there illuminated by that light, 

 and the situation of each colour within that figure. And if they please to 

 illlustrate their description with a scheme or two, it will make the business 

 plainer. By this means, if there be any difference in our way of experimenting, 

 I shall be the better enabled to discern it, and give them notice where the 

 failure is, and how to rectify it. I should be glad too, if they would favour me 

 with a description of the experiment, as it has been hitherto tried by Mr. Linus, 

 that I may have an opportunity to consider, what there is in that which makes 

 against me. 



