EYE-PRESERVING GLASS FOR SPECTACLES. 
15 
Iron (FeA). 
In this state of oxidation iron glass cuts off ultra-violet light to a limited extent. 
When small proportions only are present, such as 0'25 per cent., the rays are 
transmitted from A 3500 (far in the ultra-violet), and it is only when the amount of 
iron in the per state rises to about 2 per cent, that the glass becomes opaque to 
the rays near A 4000 (about the limit of visibility in the violet). A glass of this 
composition cuts off about 63 per cent, of the total heat. The colour is almost 
pure yellow. The glass transmits about 75 per cent, of the incident light. Iron 
in the per state, therefore, is another metal useful in combination. 
Lead. 
A plate 2 mm. thick was cut from a block of Faraday’s “heavy glass” (boro-silicate 
of lead) prepared by himself, and tested as a sample of lead glass. It is practically 
colourless and transparent, and is opaque to the ultra-violet above A 3800. Its action 
on the heat rays is slight, only cutting off about 38‘5 per cent. 
Manganese. 
Glass containing manganese is of a reddish purple colour. In respect to obstruction 
to the ultra-violet and heat rays manganese has no special action. It has, however, 
been experimented with to obtain a neutral coloured glass by adding it to glass 
containing a greenish colouring agent. 
Neodymium and Praseodymium. 
These two bodies would be useful in the quest for a suitable glass were they to be 
obtained at a price which would not be prohibitive. In aqueous solutions praseo¬ 
dymium salts are greenish yellow, and those of neodymium are of a violet-rose colour. 
Mixed together in the proportion of five parts of praseodymium to one of neodymium 
the mixture is of a neutral grey. In solid solution in glass the colours are— 
praseodymium greenish yellow, and neodymium lilac. Melted together in glass in the 
same proportion as in aqueous solution, the resulting colour is also neutral grey. In 
this respect these two elements differ from nickel and cobalt, inasmuch as the colours 
remain constant either in aqueous solution or melted in glass, and the proportion 
required to obtain neutrality of tint appears to be the same in each case. 
Uranium. 
Glasses were prepared containing from half a per cent, of uranium to over 4 
per cent. The colour of the glasses with smallest quantity of metal is very faint 
