EYE-PRESERVING GLASS FOR SPECTACLES. 
23 
The colour of Glass 252 is faint bluish green. In it copper is introduced to counter¬ 
act the yellow-brown tint given by the cerium and nickel, and to obtain a tint 
approaching neutral. It is opaque to ultra-violet rays of shorter wave-length than 
X 3680, cuts off 47 per cent, of the heat radiation, and transmits 45 per cent, of light. 
The tintometer numbers are :—Yellow, 2'0 ; blue, 4'5. 
Glass 253. 
I have already explained that black mica (biotite) is almost perfectly transparent to 
heat radiation, and at the same time opaque to the luminous rays. I thought it 
would be interesting to see what would be the effect of melting up some pieces of 
highly diathermanous biotite with soda flux. After many experiments it was found 
that a glass of a neutral tint could be made by melting biotite at a high temperature 
and fusing the result with flux. The proportions are—■ 
Haw soda flux. 88'5 
Black biotite, fused. 11'5 
lOO'O 
On testing the resulting glass it was found that it had remarkable properties in 
respect to the heat rays, but in exactly the opposite way to what was expected. It 
offers an almost complete obstruction to the invisible heat rays, and it cuts oft' 94 per 
cent, of the heat radiation, and allows 30 per cent, of the incident light to pass 
through. It is opaque to ultra-violet rays of shorter wave-length than X 3610. It is 
of a sage-green colour, and its tintometer numbers are :—Yellow, 4'0 ; and blue, 7‘0. 
Discussion of the Foregoing Results. 
I have already said that the progress of this research has widened since I commenced 
investigations three years ago. Then my object was to find a glass which would cut 
off the heat so as to preserve the eyes of those engaged in glass works. I soon found 
it difficult, even if it were advisable, to confine the research to the action on heat rays 
alone. Thus the glasses now described include specimens suitable for spectacles 
adapted to all requirements—from Eyes of Youth to Eyes of Age. 
The first necessity therefore is to find a glass which will cut off as much as possible 
of the heat radiation. Glass 246 —sage-green in colour—is almost perfect in this 
respect, as it cuts off 98 per cent, of the heat. Glass 217 —of a pale blue tint—is 
opaque to 96 per cent, of heat radiation. Next comes Glass 253 —of a neutral tint—- 
which cuts off 94 per cent., and then comes Glass 240 —-of a neutral tint—cutting off 
88 per cent., Glass 210 —of a green tint—cutting off 87 per cent, heat radiation, and 
Glass 158 —of a pale greenish yellow—cutting off 63 per cent, of the heat radiation. 
