EYE-PRESERVING GLASS FOR SPECTACLES. 
19 
Glass 202. 
I have obtained a neutral tinted glass by neutralising the orange-yellow colour 
communicated to glass by iron in the ferric state, by adding to it a little cobalt, which 
colours it blue. The composition is :— 
Soda flux. 95T5 
Fe 2 0 3 . 475 
CoS0 4 , 7II*O. 070 
100-00 
The resulting glass is opaque to ultra-violet ot shorter wave-length than X 3830, it 
cuts off 83 per cent, of the heat radiation, and transmits 25 per cent, of light. The 
tintometer numbers are :—Yellow, 2‘5 ; blue, 5‘5. 
Glass 210. 
Fused soda flux. 89*0 
Ferrous sulphate, crystallised. 8'9 
Chromic oxide. 1'3 
Carbon, in fine powder. 0*8 
ioo-o 
Glass 210 is of a bluish-green colour, the blue colour communicated by the ferrous 
oxide making the colour of the chromium not so pure a green. It cuts off all ultra¬ 
violet rays of shorter wave-length than X 3620. It obstructs 87 per cent, of the heat 
radiation, and transmits 30 per cent, of light. The tintometer numbers are :—Yellow, 
2"0 ; blue, 16"0. 
Glass 217. 
A further attempt was now made to get the iron in the ferrous state, and a glass 
was prepared of the following composition :— 
Fused soda flux. 96’80 
Ferroso-ferric oxide. 2"85 
Carbon.'.. 0"35 
100-00 
A few percentages of finely powdered carbon are added to this mixture before fusion 
to keep the iron in the proto-state. Any excess of carbon rises to the top of the fuzed 
mass and prevents oxidation of the iron. This glass is of a pale blue colour, and cuts 
off the ultra-violet rays shorter than X 3550. It cuts off 96 per cent, of the heat 
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