IX. On the Quantity of Light intercepted by a Grating placed before a 

 Lens; and on the Effect produced by Interference. By the Rev. 

 Philip Kelland, M.A., F.R.SS.L. & E. late Fellow of Queens' 

 College, Cambridge ; Professor of Mathematics in the University of 

 Edinburgh. 



[Read March 30, 1840.] 



From the remarkable appearances presented by the interception of a 

 part of the light proceeding from a small luminous body towards the object- 

 glass of a telescope, it may very naturally be supposed that vibrations are 

 suffered to exist, which would otherwise be destroyed by interference ; and 

 that consequently a less quantity of light is stopped by the grating than 

 that which is actually incident on it. That light actually appears from the 

 application of the grating, where there would be little or none without it, 

 is most certain ; and that this circumstance arises from the want of inter- 

 ference, alluded to above, there can be no doubt. Should we then expect, 

 notwithstanding the cause to which we attribute the phenomenon, (not to 

 speak of the phenomenon itself) to find exactly the same quantity of light 

 on the other side the lens, or at least in its field of view, as would corre- 

 spond to the spaces left open by the grating ? 



When first asked my opinion on this subject, I had no hesitation in pro- 

 nouncing that, previous to calculation, I should expect to find more light 

 transmitted through a grating, than in proportion to the space left un- 

 covered. My idea of the matter was this : certain vibrations are not 

 destroyed when the grating is applied, which would be destroyed in the 

 contrary case ; whereas there is nothing to affect those spaces from which 

 vibrations are excluded, so as to render this nugatory. This reasoning, 

 subsequent consideration convinces me is incorrect. It appears that, although 

 the stoppage of vibrations by the wires does bring into operation that 



Vol. VII. Part II. U 



