PHILOSOPHICAL TRANSACTIONS. 



15 



VOL. VII.] 



those parts, and of the convex 10000; supposing the sines of incidence and 

 refraction to be, in round numbers, as 2 to 3. And this table shows, how much 

 the exterior rays, at several apertures, fall short of their principal focus. 



By this you may perceive, that the errors of the refracting convex are so far 

 from being less, that they are more than sixteen times greater than the like 

 errors of the reflecting concave, especially in great apertures; and that without 

 respect to the heterogeneous constitution of light. So that, however the con- 

 trary supposition might make the authorof these animadversions reject reflections, 

 as useless for the promoting of optics; yet I must for this, as well as other con- 

 siderations, prefer them in the theory, before refractions. 



Whether the parabola be more diflicult to describe than the hyperbola or 

 ellipsis, may be a quaere : but I see no absolute necessity for endeavouring after 

 any of their descriptions. For, if metals can be ground truly spherical, they 

 will bear as great apertures, as I believe men will be able to communicate an 

 exact polish to. And for dioptric telescopes, I told you, that the difliculty con- 

 sisted not in the figure of the glass, but in the difFormity of refractions : which 

 if it did not, I could tell you a better and more easy remedy than the use of the 

 conic sections. 



Thus much concerning the practical part of optics. I shall now take a view 

 of the considerations on my theories. And those consist in ascribing an hypo- 

 thesis to me which is not mine; in asserting an hypothesis, which, as to the 

 principal parts, is not against me; in granting the greatest part of my discourse 

 if explicated by that liypothesis; and in denying some things, the truth of 

 which would have appeared by an experimental examination. 



Of these particulars I shall discourse in order. And first of the hypothesis, 

 which is ascribed to me in these words: *' But grant his first supposition, that 

 light is a body, and that as many colours or degrees as there may be, so many 

 bodies there may be; all which compounded together would make white, &c." 

 This it seems is taken for my hypothesis. It is true that from my theory I argue 



