1882.] Lord Rayleigh, On the use of Telescopes on dark nights. 197 



March 6, 1882. 



Dr Campion, Vice-President, in the chair. 



Mr C. T. Heycock, B.A., King's College, was balloted for and 

 duly elected a Fellow of the Society. 



The following communications were made to the Society: 



(!) The use of telescopes on dark nights. By Lord Rayleigh. 



In Sillimans Journal for 1881 Mr E. S. Holden, after quoting 

 observations to a like effect by Sir W. Herschel, gives details of 

 some observations recently made with a large telescope at the 

 Washburn Observatory, from which it appears that distant objects 

 on a dark but clear night can be seen with the telescope long after 

 they have ceased to be visible with the naked eye. He concludes, 

 " It appears to me that this confirmation of Herschel's experiments 

 is important, and worth the attention of physicists. So far as I 

 know there is no satisfactory explanation of the action of the 

 ordinary Night-glass, nor of the similar effect when large apertures 

 are used." 



It is a well-known principle that no optical combination can 

 increase what is called the 'apparent brightness' of a distant ob- 

 ject, and indeed that in consequence of the inevitable loss of light 

 by absorption and reflection the 'apparent brightness' is necessarily 

 diminished by every form of telescope. Having full confidence in 

 this principle, I was precluded from seeking the explanation of the 

 advantage in any peculiar action of the telescope, but was driven to 

 the conclusion that the question was one of apparent magnitude 

 only, — that a large area of given small 'apparent brightness' must 

 be visible against a dark ground when a small area would not be 

 visible. The experiment was tried in the simplest possible manner 

 by cutting crosses of various sizes out of a piece of white paper and 

 arranging them in a dark room against a black back-ground. A 

 feeble light proceeded from a nearly turned out gas-flame. The 

 result proved that the visibility was a question of apparent magni- 

 tude to a greater extent than I had believed possible. A distance 

 was readily found at which the larger crosses were plainly visible, 

 while the smaller were quite indistinguishable. To bring the latter 

 into view it was necessary either to increase the light considerably, 

 to approach nearer, or lastly to use a telescope. With sufficient 

 illumination the smallest crosses used were seen perfectly defined 

 at the full distance. 



