TOL. X.] PHILOSOPHICAL TRANSACTIONS. 26l 



with the sides thereof straight lines, just as Mr. Newton there describes 

 tliem. Neither is the length of the image transverse, but parallel to the length 

 of the prism. Out of all which it evidently follows, that the experiment was 

 not made in a clear day ; nor with the prism close to the hole ; nor yet with 

 the image transverse, as is now affirmed, but by a bright cloud, and a parallel 

 image, as I conjectured ; and I hope you will also now say, I had good reason 

 so to conjecture, since it so well agrees with the relation: and experience will 

 also show you, if you please to make trial, as it was made, in a dark chamber, 

 and observe the diiference between such an image made by a bright cloud, and 

 another made by the immediate rays of the sun : for, the former you will always 

 find parallel, with the ends semicircular; but the latter you will find transverse, 

 with the ends pyramidical as aforesaid, whenever it appears so much longer than 

 broad. 



More might be said out of the same relation, to show that the image was not 

 transverse. For if it had been transverse, Mr. Newton, so well skilled in optics, 

 could not have been surprised, as he says he was, to see the length thereof so 

 much to exceed the breadth ; it being a thing so obvious and easy to be ex- 

 plained by the ordinary rules of refraction. That other place also in the next 

 page, where he says the incident refractions were made in the experiment equal 

 to the emergent, proves again that the said oblong image was not transverse 

 but parallel. For it is impossible that the transverse image should be so much 

 longer than broad, unless those two refractions be made very unequal, as both 

 the computation according to the common rules of refraction, and experience 

 testify. Wherefore Mr. Newton had no reason to tax P. Pardies of hallucina- 

 tion, for making those two refractions very unequal : for that learned optician 

 very well saw, that in a clear day so great an inequality of length and breadth 

 could not be made, unless those two refractions were also made very unequal. 

 These places, I say, might be added to the former, and further here explained 

 if need were; but there being no need, I cease to detain you any longer 

 herein. 



Mr. Isaac Newton s Considerations on the former Reply ; together with further 

 Directions, hoiu to make the Experiments controverted aright : Cambridge, 

 November 13, 1675. N°121, p. 501. 



Sir — ^When you showed me Mr. Line's second letter, I remember I told 

 you that I thought an answer in writing would be insignificant, because the dis- 

 pute was not about any ratiocination, but my veracity in relating an experiment, 

 which he denies will succeed as it is described in my printed letters : for this is 

 to be decided not by discourse, but new trial of the experiment. What it is. 



