1882.] Dr Pearson, On the use of large telescopes in twilight. 205 



March 20, 1882. 

 Dr Campion, Vice-President, in the chair. 



Dr G. M. Bacon, M.A., was balloted for and duly elected a 



Fellow of the Society. 



(1) On the use of large telescopes in twilight. By Dr J. B. 

 Pearson. 



In his paper on this subject, read on March 6th last, Lord 

 Rayleigh stating the generally accepted law, that the brightness of 

 an object could not be increased by the use of a telescope, referred 

 to a recorded phenomenon apparently not in agreement with it. 

 I offer some remarks which perhaps may help to explain that 

 phenomenon. 



With regard to a lens the law is laid down as follows in Cod- 

 dington's Optics, pt. I. p. 219, "When the pencils of rays proceed- 

 ing from an object are received by the eye after any modification, 

 we may naturally make comparison of its apparent brightness, with 

 that of the object itself seen directly. Supposing then that the 

 rays of any pencil arriving at the eye from a lens are parallel, the 

 pencil in its original state is one which attains, at the distance of 

 the focal length of the lens from the object, a breadth equal to 

 that of the pupil of the eye. When the object is seen directly, the 

 pencil proceeding from any point has, on arriving at the eye, after 

 continually diverging, only the same breadth as the other. The 

 quantity of light therefore coming through the lens is greater than 

 that which comes to the naked eye in the duplicate ratio of the 

 distance of the eye from the object, to that of the lens from the 

 same. Now as the linear dimensions of the object are magnified 

 by the lens in the simple ratio of these distances, the surfaces are 

 also magnified in the ratio duplicate of this, that is exactly in the 

 same ratio as the quantity of light was found to be increased. The 

 object therefore should appear equally bright, with or without the 

 lens, were no light lost in passing through the glass, which how- 

 ever is always the case." 



He continues: pt. II. p. 10, "An instrument by which a faint 

 effect on the eye is multiplied and extended over the surface of 

 that organ, with as little diminution as possible of its distinctness 

 must materially assist the natural vision. Such an instrument is 

 a telescope. The nature of this advantage will perhaps appear 

 more clearly from the following consideration. A hundred flames 

 of equal magnitude and brilliancy have no more essential bright- 



