154 PROFESSOR KELLAND, ON THE 



which must otherwise have been destroyed, yet the same stoppage causes to 

 disappear certain of the vibrations corresponding with the uncovered part, 

 which would, in the contrary case, appear in the aggregate of all the 

 motion. 



My attention was called to this subject by Professor Forbes, who 

 has been prosecuting an experimental enquiry into the effect produced 

 by screens on the transmission of radiant heat. The curious fact which 

 he has established relative to the difference in amount of the stoppage 

 produced in light and dark heat, — or at least in two different kinds of heat, 

 which he has found to be operated on very differently in other matters — 

 promises to give us an insight into the characteristic properties of light 

 and heat, provided it appear that one kind of heat is, in the case before 

 us, acted on in the same manner as light is, in like circumstances. But 

 perhaps it is too much to hope that we shall distinguish betwixt light 

 and heat, uncertain as we are of the intensity of the former, by which 

 its nature might be contrasted with that of the latter. It may then be 

 expected, rather, that we shall be put in the way of distinguishing between 

 heat and undulations ; distinction being, if I mistake not, absolutely 

 necessary, as well as obviously pointed at, by the very experiments which 

 seem most strongly to identify the two with each other. 



I forbear, however, entering on this subject at present, although I 

 am deeply interested in it, as well on account of its intrinsic importance, as 

 of its bearing on my own views of the Theory of Heat. I shall therefore, 

 without further preface, proceed to the question in hand. 



Our Problem is this : — 



A series of equal parallelograms are placed before a lens, to find the 

 whole quantity of light received on a screen, placed perpendicular to the 

 axis of the lens at its focus. 



The solution of the Problem for rinding the intensity at any one in- 

 dividual point will be found in Airy's Tracts, p. 328, at the foot of 

 Art. 83. 



The expression is this : 



'!/ 2 ( 



p (e + g) 7T 

 sin t — - — r- 5 — m < 



2nqf Xb J Wpe \b I \ T p (<? + g) n 



sm Xb 



