REFRANGIBILITY of LIGHT. 6x 
nance and liberal fupport of the Royal Founder of our Society, 
the general Patron of Science. 
I APPREHEND there is a caufe which will render fhort tele- 
{copes always more diftin& than long ones, where all other cir- 
cumftances are, as nearly as poflible, alike; and that it has 
‘operated in favour of reflecting telefcopes.. It is well known 
that grofs bodies act on light at a diftance. Some phenomena 
I have obferved, appear to me to put it beyond doubt, that light 
alfo aéts upon light, in fuch a way as to propagate this action 
of grofs bodies much farther than is imagined. But I muft 
delay entering farther on this fubject ; and {hall only obferve, 
that it was principally with an eye to this circumftance, that L 
endeavoured. in my attempts to execute object-glafles on the 
above principles, to ftrgin the increafe of aperture to the ut- 
mott. 
IT will.be underftood, that when the aberrations from the 
_ difference of refrangibility of light, and from the fpherical fi- 
gures of lenfes are removed, there remains no farther limit to 
fhortening telefcopes, excepting from the requifite depth of the 
{pheres and thicknefs of the glaffes. 
I Frnp that in fmall object-glaffes of about nine inches focal 
length, the aperture may be increafed as far as three inches, 
and hardly beyond this, on account of the quick increafe of 
depth of the fpherical furfaces, and thicknefs of the glafs. 
From the difficulty found in procuring good. glafs of fufficient - 
thicknefs, it may perhaps be better to. make the aperture for 
common purpofes lefs than this. I fhall therefore ftate it at two 
inches. Hence the lengths neceflary for increafed apertures 
may readily be found, as the increafe of length is in the fame 
ratio as the increafe of aperture, a double aperture requiring a 
double length, and fo forth. Thefe lengths and apertures may 
be compared with the lengths and apertures neceflary in’ fingle 
lenfes, and. in different kinds of refleGtors, by the common. 
tables, . 
It. 
