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very valuable “ Lectures ” of Thomas Young. Kelland’s edition is 
now unfortunately, like its predecessor, entirely out of print. 
But his forte unquestionably lay in pure mathematics, and in that 
department, and others closely allied to it, he has enriched our 
“ Transactions ” with several very excellent papers. 
An idea of Professor Kelland’s scientific activity will be obtained 
by looking at the list of papers under his name in the Royal 
Society’s Catalogue of Scientific Memoirs. 
Several of his memoirs deal with physical optics. Two of these 
are especially interesting. They deal with the question of the aggre- 
gate effect of interference. In the first (“Trans. Camb. Phil. 
Soc.” vii.) he shows that when light falls on a lens, part of which 
is covered and part uncovered, the whole quantity of light on a 
screen placed in the focus is to that which falls on the lens as the 
area of the uncovered part of the glass is to the whole area of the glass. 
Hence he infers that the whole quantity of light is not diminished 
or increased by interference. In the second (“ Trans. R.S.E.” xv.), 
starting from the principle thus established, he treats a very interest- 
ing point which arises in the application of Huyghens’ principle in 
the undulatory theory. In forming the expression for the vibration 
due to any element of an aperture on the surface of a lens, we 
multiply the maximum intensity of vibration by the area of the 
element, and to keep the dimension correct we must divide by a 
factor D whose dimension is the square of a line. Kelland investi- 
gates a variety of cases for different forms of aperture, and finds in 
each case that D must be bX where X is the wave-length of the 
incident light, and b the distance from the lens of the screen placed 
in its focus. The question was afterwards discussed by Stokes 
(“Trans. R.S.E.” xx.), who generalised Kellands analysis, and 
showed that the result may be deduced for an aperture of any 
form. 
In a memoir read before the Royal Society of Edinburgh in 
April 1839 (“Trans.” xiv.) Kelland took up the subject of wave 
motion. He discusses the case of a canal of finite depth h , 
adopting the hypothesis of parallel sections. Assuming the motion 
to be undulatory, and taking 
z = li + a sin (pi - x ) 
A 
