24 
SIR WILLIAM CROOKES ON THE PREPARATION OF 
If more light is desired to enable the workers to see better at the expense of a little 
athermancy, Glass 249 is commendable. It is a very pale blue and transmits 63 per 
cent, of luminous rays while cutting off 51 per cent, of the heat rays. Spectacles of 
this glass scarcely appear to obstruct light at all, and the colours of objects are 
practically unchanged. 
For ordinary use, when no special protection against heat radiation is needed, the 
choice will rest on whether the ultra-violet or the luminous are most to be guarded 
against ; or whether the two together are to be toned down. To begin with I will 
take into consideration glasses most effective in cutting off ultra-violet rays. 
Ordinarily the visible spectrum is assumed to end at the Fraunhofer line K, A 3933, 
but light can easily be distinguished some distance beyond by the naked eye. For 
instance, about fifty years ago, with a quartz spectroscope with solar rays, I could see 
L, A 3820, and M, A 3727, though now I cannot see above K. It may therefore be 
considered that the ultra-violet rays which are to be cut oft’ on account of their 
probable injurious action are those of shorter wave-lengths than, say, A 3700. The 
most effective glasses for this purpose are 158, 150, 240, 246, 202, and 197, all of 
which are opaque to rays shorter than 3700. The colours are pale green, yellow and 
neutral, they transmit ample light so that a choice of tints is available to suit 
individual taste. 
If much transparency is required there is a choice between glasses 187, 150, 251, 
250, 247, and 238, which transmit from 99'5 to 70 per cent, of the incident light. 
The choice between this range of glasses will depend on the conditions required, i.e., 
on the absolute transparency or the colour. The colours are pale tint of yellow, green, 
and neutral. 
If only a moderate degree of transparency is desired Glasses 158, 249, and 221 may 
be selected; the light transmitted ranges from 70 to 60 per cent., and the tints are of 
pleasant green, blue, and orange. 
When glasses are required which are restful to the eyes in the glare of the sun on 
chalk cliffs, expanses of snow, or reflected from the sea, Glasses 249, 197, 252, 165, 
210, and 248 are most suitable, the tints being yellow, green, and neutral. Moreover, 
they have the advantage of cutting oft* practically all the ultra-violet rays and also 
a considerable amount of the heat radiation. 
For convenience of reference all the glasses above described are arranged in the 
following four tables :— 
I. Absorption of Heat Fays. 
II. Absorption of Ultra-violet Fays. 
III. Transmission of Luminous Fays. 
IV. Feduction of Glare. 
This latter property is the most valuable for general use in brilliant light. 
