REFRANGIBILlTr of LIGHT. 61 



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- 

 fcopes always more diltinct than long ones, where all other cir- 

 cumftances are, as nearly as poffible, 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 acts 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 fhall only obferve, 

 that it was principally with an eye to this circumftance, that I 

 endeavoured in my attempts to execute object-glaffes on the: 

 above principles, to ftrain the increafe of aperture to the ut- 

 mofl. 



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 requisite depth of the 

 fpheres and thicknefs of the glafTes. 



I ftnd 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 neceffary 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 neceffary in fingle 

 lenfes, and in different kinds of reflectors, by the common 

 tables. 



It. 



