Ptnetralton into space by Telefcoffis. 55 (. 



Firft, with regard to the eye, it is certain that its power, lilce all our other facuUies, is 

 Hmited by natUfe, and is regulated by the permanent brightnefs of objects ; as has been 

 fliewn already, when its extent with refle€led light was compared to its exertion on felf- 

 luminous objeifls. It is further limited on borrowed light, by the occafional ftate of illu- 

 mination ; for, when that becomes defeftive at any time, the power of the eye will then 

 be contrafled into a narrower compafs ; an inftance of which is the following : 



In the year 1776, when I had ereded a telefcope of 20 feet focal length, of the New- 

 tonian conftrudlion, one of its eftefts by trial was, that when towards evening, on account 

 of darknefs, the natural eye could not penetrate far into fpace, the telefcope poffeffed that 

 power fufficiently to fhew, by the dial of a diftant church fteeple, what o'clock it was, 

 notwithftanding the naked eye could no longer fee the fteeple itfelf. Here I only fpeak of 

 the penetrating power ; for, though it might require magnifying power to fee the figures 

 on the dial, it could require none to fee the fteeple. Now the aperture of the telefcope 

 being 12 inches, and the conftruftiop of the Newtonian form, its penetrating powcr^ 



when calculated according to the given formula, will be ^ ''^ 9 ^ ■'^ ir 38,99. 



2 



yi, b, and a,, being all expreiTed in tenths of an inch *. 



From the refult of this computation it appears, that the circumftance of feeing fb well^ 

 in the dufk of the evening, may be eafily accounted for, by a power of this telefcope to 

 penetrate 39 times farther into fpace than the natural eye could reach, with objedls fo 

 faintly illuminated. 



This obfervation completely refutes an objeflion to telefcopic vifion, that may be drawn 

 from what has alfo been demonftrated by optical writers : namely, that no telefcope can 

 fliew an objeft brighter than it is to the naked eye. For, in order to reconcile this optical 

 theory with experience, I have only to fay, that the objeclion is intirely founded on the 

 fame ambiguity of the word brightnefs that has before been dete£led. It is perftftly true-, 

 that the intr'mfic illumination of the piflrure on the retina, which is made' by a telefcope, 

 cannot exceed that of natural vifion ; but the nbfolute brightnefs of the magnified pi£luri 

 by which telefcopic vifion is performed, mult exceed that of the pifture in natural vifion, 

 in the fame ratio in which the area of the magnified pidure exceeds that of the natural 

 one ; fuppofing the intrmfic brightnefs of both pictures to be the fame. In our prefent 

 inftance, the fteeple and clock-dial were rendered vifible by the increafed abfolute bright- 

 nefs of the obje£l, which in natural vifioii was 1$ hundred times infeHor to v^hafit was in 

 the telefcope. And this eftablifhes beyond a doubt, thit telefcopic vifion is performed by 

 the abfolute brightnefs of objefls ; for, in the prefent cafe, I find by computation^ that 

 the Intrhific brightnefs, fo far from being equal in the telefcope to that of natural' vifion, 

 was inferior to it in the ratio of three' to feven. 



* I have given the figures, in all the following equations of the calculated penetrating' powers, in order 

 to (hew the conftruflions of my inftruments to thofc who may wilh to be acquainted with them. 



The 



