\ 
300 Intelligence and Miscellaneous Articles. 
corresponding element of the vertical column, and sets the 
tune vertical in a position corresponding to that of the column. 
When these operations are prefixed to those which he ascribes 
to Lord Rayleigh, a series of processes is obtained which, of 
course, might as well be terminated after the first, i. e., the 
partial separation of the gases in the horizontal tube ; and 
Mr. Burbury contends that Lord Rayleigh has proved 
nothing inconsistent with the supposition that this separation 
—at constant pressure and constant temperature — can be 
performed without the expenditure of work. In one sense 
this contention may be admitted, for, if I apprehend the 
situation correctly, Lord Rayleigh's investigation is not 
directly relevant to the series of processes which Mr. Burbury 
describes. If, however, in the reservoir, a small portion of 
gas of the same constitution as that at the top of Lord Ray- 
leigh's tube be separated from the remaining contents, and 
if this partial separation can be performed at constant 
temperature and constant pressure without doing any work 
oh the gases,. then, by reversing the processes which Lord 
Rayleigh, if I understand correctly, actually does employ, it 
follows from his investigation that the gas system would 
again constitute the working substance in a heat engine 
which contradicts the Second Law of Thermodynamics. 
XXVIII. Intelligence and Miscellaneous Articles. 
OX THE FOCOMETRY OF A CONCAVE LEXS. 
To the Editors of the Philosophical Magazine. 
Gentlemen, — 
TT has just been pointed out to me that the first method I de- 
-*- scribed in the " Note on the Focometry of a Concave Lens " 
in the Philosophical Magazine for January, is given in the Syllabus 
of the Course of Practical Instruction in Physics at the Eoyal 
College of Science, London (1902). 
I was, unfortunately, unaware of this at the time of writing, as 
it is not given in my copy, which is an earlier edition (1892). 
The second method is, of course, similar in principle to the first ; 
while the third is derived from the second by reversing the direction 
of the rays. 
Yours faithfully, 
J. A. Tomkixs. 
