VOL. VIII.] PHILOSOPHICAL TRANSACTIONS. QS 



that. But I believe there cannot be found an experiment of that kind ; be- 

 cause as I remember I once tried, by gradual succession, the mixture of all 

 pairs of uncompounded colours ; and, though some of them were paler, and 

 nearer to white than others, yet none could be truly called white. But it being 

 some years since this trial was made, I remember not well the circumstances, 

 and therefore recommend it to others to be tried again. 



In the last place, had I thought the distinctness of the picture, which, for in- 

 stance, a 12 foot object-glass casts into a darkened room, to be so contrary to 

 me as N. is pleased to affirm, I should have waved my theory in that point, be- 

 fore I propounded it. For, that I had thought on that difficulty, you may 

 easily guess by an expression, somewhere in my first letter (N° SO) to this pur- 

 pose ; that I wondered how telescopes could be brought to so great perfection 

 by refractions, which were so irregular. But to take away the difficulty, I must 

 acquaint you first, that though I put the greatest lateral error of the rays from 

 one another, to be about -^ of the glass's diameter; yet their greater error from 

 the points on which they ought to fall, will be but -ris- of that diameter : and 

 then that the rays, whose error is so great, are but very few in comparison to 

 those which are refracted more justly; for, the rays which fall upon the middle 

 parts of the glass, are refracted with sufficient exactness, as also are those that 

 fall near the perimeter, and have a mean degree of refrangibility ; so that there 

 remain only the rays which fall near the perimeter, and are most or least re- 

 frangible, to cause any sensible confusion in the picture. And these are yet so 

 much further weakened by the greater space, through which they are scattered, 

 that the light which falls on the due point, is infinitely more dense than that 

 which falls on any other point round about it. Which though it may seem a 

 paradox, yet is certainly demonstrable. Yea, although the light, which passes 

 through the middle parts of the glass, were wholly intercepted, yet would the 

 remaining light convene infinitely more dense at the due points, than at other 

 places. And by this excess of density, the light which falls in or invisibly near 

 the just point, may, I conceive, strike the sensorium so vigorously, that the 

 impress of the weak light, which errs round about it, shall, in comparison, not 

 be strong enough to be animadverted, or to cause any more sensible confusion 

 in the picture than is found by experience. 



This I conceive is enough to show why the picture appears so distinct, not- 

 withstanding the irregular refraction. But if this satisfy not, N. may try, if he 

 please, how distinct the picture will appear, when all the lens is covered except- 

 ing a little hole next its edge on one side only : and if in this case he please to 

 measure the breadth of the colours, thus made at the edge of the sun's picture, 

 he will perhaps find it to approach nearer to my proportion than he expects. 



