penetrating into Space by Telescopes. 55 



near so', before it could be sufficiently reposed to admit a view 

 of very delicate objects in the telescope ; and that the observa- 

 tion of a transit of a star of the 2d or 3d magnitude, would 

 disorder the eye again, so as to require nearly the same time 

 for the re-establishment of its tranquillity. 



The difficulty of ascertaining the greatest opening of the eye, 

 arises from the impossibility of measuring 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 easily to be 

 ascertained, we have, on the other hand, no difficulty to deter- 

 mine the quantity of light admitted through a telescope, which 

 must depend upon the diameter of the object-glass, or mirror ; 

 for, its aperture A may at all times be had by measurement. 



a 2 " I 



It follows, therefore, that the expression -^ will always be 

 accurate for the quantity of light admitted by the eye ; and that 



A 1 1 



-jp will be sufficiently so for the telescope. For it must be 



remembered, that the aperture of the eye is also concerned in 

 viewing with telescopes ; and that, consequently, whenever the 

 pencil of light transmitted to the eye by optical instruments 

 exceeds the aperture of the pupil, much light must be lost. In 

 that case, the expression A x I will fail ; and therefore, in gene- 

 ral, if m be the magnifying power, — ought not to exceed a. 



As I have defined the brightness of an object to the eye of 

 an observer at a distance, to be expressed by ■—, it will be 



necessary to answer some objections that may be made to this 

 theory. Optical writers have proved, that an object is equally 

 bright at all distances. It may, therefore, be maintained against 

 me, that since a wall illuminated by the sun will appear equally 



