371 



Finally, tho author inferred from the whole inquiry, 1. That se- 

 cretion is a function of the nucleated cell, and takes place within it ; 

 and, 2. That growth and secretion are identical — the same process 

 under different circumstances. 



The following gentlemen were duly elected Ordinary Fel- 

 lows of the Society : — James Millar, Esq. F.R.C. Surg. ; Sir 

 James Forrest, Bart. Lord Provost of Edinburgh ; James 

 Stark, M.D., F.R.C. Phys. ; John Adie, Esq. Optician. 



Monday, Alh April 1842. 



Sir T. M. BRISBANE, Bart., President, in the Chair. 



1. On the Theoretical Investigation of the Absolute Intensity 

 of Interfering Light. By Professor Kelland. 



This Memoir is the prosecution of a subject on which the author 

 had previously touched in a paper which is printed in the seventh 

 volume of the Transactions of the Cambridge Philosophical Society. 

 It has for its object the investigation of the quantity of light which 

 is received on a screen of unlimited dimensions, after passing through 

 a certain aperture, or suffering reflexion at two mirrors. The end 

 for which the investigation is undertaken is to ascertain the constant 

 which must be introduced in using Huygens's principle. From the 

 fact, that each vibration at the aperture is multiplied by an area, on 

 finding the effect on the screen, it is evidently i-equisite that a divi- 

 sor of two dimensions is required to render the vibration at a point 

 on the screen, similarly expressed with that at the aperture. Ac- 

 cording to the author's investigation, it appears that the divisor is 

 the product of the length of a wave, and the perpendicular distance 

 between the screen and the aperture. Hence, in reference to such 

 questions, the enunciation of Huygens's principle is as follows : — 



The libration, at a given point, caicsed by a given wave, is found 

 by taking the front of the wave, dividing it into an indefinite num- 

 ber of small parts, considering the agitation of each of these parts 

 as the origin of a wave whose maximum of vibration, on reaching 

 the point, is equal to the quotient of that at the disturbing point, 

 divided by the product of the length of the ivave, and the perpendi- 

 cular from the disturbed point on the front of the wave. 



