44, PRESIDENT’S ADDRESS. 
Sulphur and arsenic compounds are the most opaque; 
boron and aluminium compounds are, as a rule, amongst the 
most transparent. Boric anhydride is even more transparent 
than the diamond. 
He has framed a scale of transparency for minerals :— 
1, Diamond ; 2, Corundum; 3, Talc; 4, Quartz; 5, Rock- 
salt; 6, Calcite; 7, Cerussite; 8, Realgar, which is quite 
opaque. 
Crystals only show slight differences when viewed in 
various directions. 
COLOUR PHOTOGRAPHY. 
A great deal of interest is taken in this subject, and 
repeated statements have been made durimg the last year 
or two that success had at last been obtained; but up to 
the present no one has succeeeed in taking and fixing 
photographs in their natural colours, and on a single plate, 
or by a single print. Mr. Ives takes three transparencies 
from the same subject—one through an orange, one through 
a green, and one through a blue screen ;—each is then 
illuminated by the corresponding light, and the three 
images superimposed, when a picture is presented im its 
natural colours. 
Dr. Joly, of Dublin, reproduces the colour by a single 
negative by means of closely ruled red, blue, and green 
coloured lines on revolving screens; but the colours are 
only seen when the viewing screen isused. Another method 
is to use coloured transparent inks (printed from three 
negatives, taken by red, blue, and green lights respectively), 
instead of coloured films. 
Becquerel found, fifty years ago, that if a plate be 
chloridised instead of iodised, and then exposed to white 
light, it acquired a violet tint, and that in this condition, 
on exposure to the solar spectrum, all the colours were 
reproduced, but unfortunately they cannot be fixed. 
Lippman has found that by reflection from a film of 
mercury at the back of the negative he can obtain stationary 
