Penelralion into Space by Tele/copes. 499 



tclefcope, at the time of fweeping, was no more than , 1 2 inch. The effe£t of this increafed 

 fenfibllity was fuch, that if a ftar of the 3d magnitude came towards the field of view, I 

 found it neceflary to withdraw the eye before its entrance, in order not to injure the deli- 

 cacy of vifion acquired by long continuance in the dark. The tranfit of large ftars, unlefs 

 where none of the 6th or 7th magnitude could be had, have generally been declined in my 

 fweeps, even with the 20 feet telefcope. And I remember, that after a confiderable fweep 

 with the 40 feet inftrument, the appearance of Sirius announced itfelf, at a great diftancc» 

 like the dawn of the morning, and came on by degrees, increafing in brightnefs, till this 

 brilliant ftar at laft entered the field of view of the telefcope, with all the fplendour of the 

 rifing fun, and forced me to take the eye from that beautiful fight. Such ftriking effcdls 

 are a fufiicient proof of the great fenfibility of the eye, acquired by keeping it from the 

 light. 



On taking notice, in the beginning of fweeps, of the time that pafled, I found that the 

 eye, coming from the light, required near 20', before it could be fufficiently repofed to ad- 

 mit a view of very delicate objedts in the telefcope ; and that the obfervation of a tranfit of 

 a ftar of the 2d or 3d magnitude, would diforder the eye again, fo as to require nearly the 

 fame time for the re-eftablifhment of its tranquillity. 



The difficulty of afcertaining the greateft opening of the eye, arifes from the impofli- 

 bility of meafuring it at the time of its extreme dilatation, which can only happen when 

 every thing is completely dark ; but, if the variation of a is not eafily to be afcertained, we 

 have, on the other hand, no difliculty to determine the quantity of light admitted through a 

 telefcope; which muft depend upon the diameter of the obje£t-glafs, or mirror ; for, its 

 aperture ji may at all times be had by meafurement. 



It follows, therefore, that the exprelfion -^^ will always be accuTrate for the quantity of 



A"/ 

 light admitted by the eye ; and that ^^ will be fufficiently fo for the telefcope. For it 



muft be remembered, that the aperture of the eye is alfo concerned in viewing with telef- 

 copes ; and that, confequently, whenever the pencil of light tranfmitted to the eye by op- 

 tical inftruments exceeds the aperture of the pupil, much light muft be loft. In that cafe, 

 the expreffion ^^ / will fail ; and therefore, in general, if m be the magnifying power, 



— ought not to exceed a. 

 As I have defined the brightnefs of an obje£l to the eye of an obferver at a diftance, to be 



Cxprefled by -^^, it will be neceflary to anfwer fome objeftions that may be made to this 



theory. Optical writers have proved, that an obje£l is equally bright at all diftances. It 

 may, therefore, be maintained againft me, that fince a wall illuminated by the fun will ap- 

 pear equally bright, at whatfoever diftance the obferver be placed that views it, the fun alfo, 



3S 2 at 



