714 PHILOSOPHICAL TRANSACTIONS. [aNNO 16^2, 



Gregory himself, at London, caused one of 6 feet to be made by Mr. Reive, 

 which I take to have been according to the aforesaid design described in his 

 book ; because, though made by a skilful artist, yet it was without success. 



I could wish therefore, M. Cassegrain had tried his design, before he divulged 

 it : but if, for further satisfaction, he please hereafter to try it, I believe the 

 success will inform him, that such projects are of little moment till they be put 

 in practice. 



Some Experimetifs proposed in relation to Mr. Newton's Theory of 

 light, printed in N° 80; with the Observations made thereupon hy 

 the Author of that Theory , communicated in a Letter of his from 

 Cambridge, April IS, 1672. N'8S,p.d059. 



1 . To contract the beams of the sun without the hole of the window, and to 

 place the prism between the focus of the lens and the hole, spoken of in M. 

 Newton's theory of light. 



2. To cover over both ends of the prism with paper at several distances from 

 the middle ; or with moveable rings, to see how that will vary or divide the 

 length of the figure, insisted upon in the said theory. 



3. To move the prism so, as the end may turn about, the middle being 

 steady. 



4. To move the prism by shoving it, till first the one side, then the middle, 

 then the other side pass over the hole, observing the same parallelism. 



The Observations made upon these Proposals. 



I suppose the design of the proposer of these experiments is, to have their 

 events expressed, with such observations as may occur concerning them. — 

 1 . Touching the first, I have observed, that the solar image falling on a paper 

 placed at the focus of the lens, was by the interposed prism drawn out in length 

 proportional to the prism's reflection or distance from that focus. And the chief 

 observable here, which I remember was, that the straight edges of the oblong 

 image were more distinct than they would have been without the lens. 



Considering that the rays coming from the planet Venus are much less in- 

 clined one to another, than those which come from the opposite parts of the 

 sun's disk ; I once tried an experiment or two with her light. And to make it 

 sufficiently strong, I found it necessary to collect it first by a broad lens, and 

 then interposing a prism between the lens and its focus, at such distance that 

 all the light might pass through the prism, I found the focus, which before 

 appeared like a lucid point, to be drawn out into a long splendid line by the 



