VO L. XI.] PHILOSOPHICAL TRANSACTIONS. 277 



served for the demonstration, had the distance been put twenty times greater 

 than it is. In the schemes of the experiment N° 80 and N° 82, it is repre- 

 sented close, and close enough in the scheme N° 83. But Mr. Linus thought 

 fit to wink at these, and pitch upon the scheme of a demonstration, and such 

 a scheme too as has no hole at all represented in it. For the scheme, N® 84, 

 (fig. 5. pi. 9) is this; in which the rays are not so far distant from one another 

 at GL, but that the hole, had I expressed it, might have been put there, and 

 yet have comprehended them. But if we should put the hole at x, their de- 

 cussation, yet will it not be any thing to his purpose ; the distance xG or xL 

 being but about half the breadth of a side of the prism (^AC), which I con- 

 ceive is not the 20th part of the distance requisite in his conjecture. 



3. He says, that " more might be said out of my relation to show that the 

 image was not transverse; for if it had been transverse, I could not have been 

 surprised, as I said I was, to see the length thereof so much exceed the breadth, 

 it being a thing so obvious and easy to be explicated by the ordinary rules of re- 

 fraction." But on the contrary, it may rather be said, that if the image had 

 been parallel, I could not have been surprised to see the length thereof so much 

 exceed the breadth, it being a thing so extremely obvious as not to need any 

 explication. For who that had but common sense, and saw the whole prism, 

 or a good part of it, illuminated, could not expect the light should have the 

 same long figure upon the wall that it had when it came out of the prism .'' Air. 

 Linus, therefore, while he would strengthen his argument by representing me 

 well skilled in optics, does but overthrow it. But whereas, he says, " I could 

 not have been surprised at the length, had the image been parallel, it being a 

 thing so obvious and easy to be explicated by the ordinary rules of refraction ;" 

 let any man take the experiment entire as I have there delivered it, that is, 

 with this condition^ that the refractions on both sides the prism were equal, and 

 try if he can reconcile it with the ordinary rules of refraction. On the contrary, 

 he may find the impossibility of such a reconciliation demonstrated in my an- 

 swer to P. Pardies, N° 84. 



In the last place he objects, that my saying in N° 80, " that the incident re- 

 fractions were, in the experiment, equal to the emergent," proves again, that 

 the long image was parallel. And yet that very saying is a sufficient argument 

 that I meant the contrary, because it becomes wholly impertinent, if applied to 

 a parallel image ; but in the other case is a very necessary circumstance. What 

 is added, therefore, of P. Pardies, might have been spared, especially since that 

 learned person understood my discourse to be meant of a transverse image, and 

 acquiesced in my answers. 



This in answer to Mr. Linus's letter: and now, to take away the like suspicions 



