164 
having perjured himself may be justifiable, and such a sus- 
picion may possibly be removed from an innocent person by 
his producing for chemical examination a paper written 
about the same time and with the same ink as that said to 
have been used in writing the letter in question. 
I have arranged on two sheets of paper the writings on 
envelopes of fifty different persons, lately sent to me. One 
sheet contains 24, written in Manchester and the suburbs, 
and the other contains 26 from London and the provinces ; 
and from a minute inspection of these it will be observed 
that most of them are very different from each other, whilst 
no two give exactly similar shades of colour with all the 
different reagents. 
PHYSICAL AND MATHEMATICAL SECTION. 
April 13 th, 1880. 
E. W. Binney, F.R.S., F.G.S., President of the Section, in 
the Chair. 
Colorimetry, Part lY.,” by James Bottomley, D.Sc. 
On the Colour Relatioiis of Nickel and Cohalt 
For some experiments which I was making in colori- 
metry, I wished to obtain a solution which would absorb 
all the kinds of light in the same ratio, so that whatever 
sort of light we started with, after penetration through 
such a solution, it would remain the same in character; 
the only variation being a change in intensity. Hence 
through such solutions white surfaces would appear grey of 
