4 
SIR WILLIAM CROOKES ON THE PREPARATION OF 
A careful examination of the six photographs shows a general progressive character, 
the extent of spectrum photographed extending into the ultra-violet as the length of 
exposure increases. 
The extent of spectrum into the region of the ultra-violet is conveniently shown in 
the following tabular form* :— 
No. 1 photograph, exposed 20 minutes, extends to A 4520. 
5 5 
55 
2 
3 
4 
5 
6 
55 
55 
45 „ „ A 4320. 
30 „ „ A 3790. 
60 „ „ A 3640. 
120 „ „ A 3595. 
180 „ „ A 3345. 
Taking the ordinary limit of visibility to lie between A 3900 and A 7600, it is seen 
that with an exposure of three hours to the highest heats the strength of impression 
does not extend much into the ultra-violet. The heat rays are very strong, and if 
injury to the eye is caused by exposure to radiation from the molten glass, a protective 
glass should be opaque to the infra-red rays. 
These being present in the radiation from molten glass in far greater abundance 
than the ultra-violet rays, the inference is that it is to the heat rays rather than to 
the ultra-violet rays that glass workers’ cataract is to be ascribed. It is, however, 
certain that exposure to excess of ultra-violet light also injuriously affects the eye. 
That the ultra-violet rays act on the deeper-seated portions of the eye is shown by 
the intense fluorescence of the crystalline lens induced by these rays. 
Besides the invisible rays at each end of the spectrum, the purely luminous rays, if 
present in abnormal intensity, are found to damage the eye. It therefore would be an 
advantage if in addition the obscuring glass for the spectacles were to be of a neutral 
or grey tint. 
Synthetic Preparation of Glasses. 
It soon became evident that my best, if not only, chance of solving the problem was 
to make different glasses in my own laboratory, with the addition of known quantities 
of pure metallic oxides and earths as colouring or absorbing materials. Lapidary 
* In connection with this table the following scale for correlating colours with wave4engths will be 
useful:— 
Wave-lengtbs 7230 and below = infra-red. 
From A 7230 to A 6470 — red. 
,, A 6470 ,, A 5850 = orange. 
„ A 5850 „ A 5750 = yellow. 
,, A 5750 ,, A 4920 = green. 
,, A 4920 ,, A 4550 = blue. 
„ A 4550 ,, A 4240 = indigo. 
A 4240 ,, A 3970 = violet. 
A 3970 and above = ultra-violet. 
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