212 Newton's Letters, Hypothesis and Experiments 



which pass very near the edge being thereby made to stray 

 at all angles into the shadow of the knife. 



To this Sir William Petty, then president, returned a very 

 pertinent query, whether that straying was in curved lines ? 

 and that made me, having heard Mr. Hook some days before 

 compare it to the straying of sound into the quiescent medium, 

 say, that I took it to be only a new kind of refraction, caused 

 perhaps by the external aether's beginning to grow rarer a little 

 before it came at the opake body, than it was in free spaces, 

 the denser aether without the body, and the rarer within it, 

 being terminated not in a mathematical superficies, but pass- 

 ing into one another through all intermediate degrees of den- 

 sity : whence the rays, that pass so near the body, as to come 

 within that compass where the outward aether begins to grow 

 rarer, must be refracted by the uneven denseness thereof, and 

 bended inwards toward the rarer medium of the body. To 

 this Mr. Hook was then pleased to answer, that though it 

 should be but a new kind of refraction, yet it was a new one. 

 What to make of this unexpected reply I knew not, having 

 no other thought, but that a new kind of refraction might be 

 as noble an invention as anything else about light; but it made 

 me afterward, I know not upon what occasion, happen to say, 

 among some that were present to what passed before, that I 

 thought I had seen the experiment before in some Italian 

 author. And the author is Honoratus Faber, in his dialogue 

 De Lumine, who had it from Grimaldo ; whom I mention, 

 because I am to describe something further out of him, which 

 you will apprehend by the opposite figure. Suppose the sun 

 shine through the little hole H K into a dark room upon the 

 paper PQ, and with a wedge MNO intercept all but a little 

 of that beam, and you will see upon the paper six rows of 

 colours, R, S, T, V, X, Y, and beyond them a very faint light 

 spreading either way, such as rays broken, like HNZ, must 

 make. The author describes it more largely in divers schemes. 

 I have time only to hint the sum of what he says. 



Now for the breaking of the ray HNZ, suppose, in the 

 next figure, MNO be the solid wedge, ABC the inward 

 bound of the uniform rarer aether within, between which 

 bounds the aether runs through all the intermediate degrees; 

 and it is manifest, that, if a ray come between B and N, it 

 must in its passage there bend from the denser medium to- 

 wards C, and that so much the more, by how much it comes 

 nearer N. Further, for the three rows of colours VXY, those 

 ma}' perhaps proceed from the number of vibrations (whether 

 one, two, or three) which overtake the ray in its passage 

 from G, till it be about the midway between G and H, that 



