EYE-PRESERVING GLASS FOR SPECTACLES. 
13 
I will now take the metals selected for further trials, and give the results of the 
preliminary test of the glasses so as to ascertain their behaviour in the four instruments 
above described. 
Cerium. 
One of the most important additions to soda flux is ceria, which gives a practically 
colourless glass. Cerium nitrate was generally used, and occasionally cerium borate 
and ceric oxide. Trial glasses were made, the proportion of metal varying from 1 per 
cent, to 7‘5 per cent. The conclusion arrived at on tabulating and considering the 
results shown by this series of glasses, is that cerium is of value in cutting off the 
ultra-violet rays. The glasses are very slightly coloured, and allow nearly all the 
luminous rays to pass. The heat absorption is about 30 per cent., and does not vary 
much with the amount of cerium present. 
Chromium. 
This metal, in quantities of less than 1 per cent, in the glass, exerts a strong action 
on the ultra-violet rays, cutting them off down to the blue (X 4550). In larger 
proportions, either singly or mixed with other metals, the absorption extends as far 
as X 5600 (about the middle of the green). Its heat obstructing power is not on a par 
with that of uranium, being about 30 per cent, for 1 per cent of chromium metal. 
The luminous rays transmitted by chromium glass containing 0'85 per cent, of metal 
are 37 per cent, of the total light, the colour of the glass being green. 
Cobalt and Nickel. 
Cobalt colours glass a rich blue, and then transmits ultra-violet rays of shorter 
wave-length than about X 3200. It cuts off 40 per cent, of the heat, and unless in very 
small quantity obstructs too much light to be of use. Nickel colours glass brown. 
In glass its absorption of ultra-violet light is about the same as cobalt. It obstructs 
a little more heat and is more transparent to light. These two metals separately are 
of no use for the present purpose, but united they have the valuable property of 
neutralising each other’s colour and giving the glass a neutral grey tint. 
It is noteworthy that the colours of nickel and cobalt in aqueous solution are green 
and pink, whilst in solid solution in glass they are brown and blue, in each case 
complementary to one another. 
Solutions of nickel and cobalt sulphates, containing 5 gr. to 100 c.c. of water, mixed 
together in the proportion of 2'5 c.c. Ni to 1 c.c. of Co, gave a mixture of a neutral 
grey colour. The mixture was divided into two parts ; one was gradually heated to 
the boiling-point, while the other was left for comparison at the temperature of the 
laboratory (16° C.). Compared with the cold solution, the one at the boiling point 
