250 
SIR J. COXROY OY THE AMOUNT OF LIGHT REFLECTED 
The surfaces of the glass plates were always cleaned with a wash-leather imme¬ 
diately before the plates were used, but, in order to ascertain whether this was 
sulhcient, a number of readings were made with a piece of plate glass treated in this 
way, and then the plate was cleaned with strong nitric acid, washed with water, 
with alcohol, and again with water, dried with a clean cloth, and finally rubbed with 
a wash-leather. The means of the twelve readings made immediately before, and 
immediately after, the plate had been so treated were identical. 
Table I. gives the readings made with the 24’3 mm. plate of crown glass. The 
observations made with the other pieces of glass were about as concordant. 
two positions of the photometer in which there is equality x-^ and x^, and the coefficient of transparency 
for the i^articular piece of glass t, then 
then 
mlc 
(a: — 
h - 
and 
nh 
m {x — 
7,2_ x-^^ (x 
x,^ {x — .rj)- ’ 
7. _ H {x — x„) 
(X — X2)^ 
and7.= llihJZi^, 
nxc,- 
{x aq} 
The difference, however, between the values of h obtained by taking’ the quotient of the squares of the 
geometrical means of the readings, and by taking the quotient of the squares of the arithmetical means, 
+ (x — aolX 
.*3 + (x — * 1 ) 
2 
was very small, and, as the readings themselves could not be made with any very great degree of 
accuracy, the simpler pi’ocess was used for calculating out the results. 
