12 
SIR WILLIAM CROOKES ON THE PREPARATION OF 
To be generally useful, it is desirable to obtain a glass which will absorb rays of 
longer wave-length than about X 7200, and so cut off dark heat radiation. It should 
also be opaque to wave-lengths shorter than about A 3550, thus cutting off the most 
chemically active rays, and also those which give rise to ionisation, i.e., cause the air 
through which they pass to conduct electricity. 
Working in a vacuum and with sensitive plates of emulsion containing no gelatine, 
Dr. Schumann succeeded in photographing ultra-violet rays as short as A 1000. In 
a paper recently read before the French Physical Society by MM. Karl Stockhausen 
and Fritz Schanz, it is stated that the harmful action of light on the eye is due to 
the ultra-violet rays. It is also shown that the cornea is opaque to rays shorter than 
A 3200. 
The crystalline lens is opaque to rays shorter than A 3500, and rays of longer wave¬ 
lengths than this reach the retina. As the transparency of ordinary spectacle glasses 
is limited to A 3000, it follows that a considerable amount of ultra-violet radiation 
may reach the cornea through ordinary spectacles. 
Method of Tasting (Hass. 
Single metals were at first tried in varying quantities to see if from the colour and 
properties communicated to the glass they were worth further examination. Each 
specimen is cut and polished into a plate 2 mm. thick. The plate so prepared is first 
tested in the spectrum apparatus to ascertain the upper limit of transmission of the 
ultra-violet rays. It is next put into the radiometer balance to see the percentage of 
heat cut off, then tested in Chapman Jones’s opacity balance* to see the percentage 
of luminous rays transmitted, and finally the colour is registered in a Lovibond’s 
tintometer, f 
A large proportion of the known metallic elements were tested in this manner, and 
a considerable number were proved to be unsuitable. After experiments extending 
over several months the following elements were selected as likely to be worthy of 
further experimentation by combining the metals two, three, or four at a time in one 
glass so as to enable the advantages of one to make up for the shortcomings in 
another:— 
Cerium. 
Manganese. 
Chromium. 
Neodymium. 
Cobalt. 
Nickel. 
Copper. 
Praseodymium. 
Iron. 
Uranium. 
Lead. 
* “An Opacity Balance,” by Chapman Jones, ‘The Photographic Journal,’ vol. xxiii., p. 99. 
t The tintometer is an instrument devised by Mr. Lovibond (Messrs. Gallenltamp and Co.). Any 
colour can be matched by a combination of three sets of glasses, coloured respectively red, yellow, and 
blue, and numbered in order of depth of colour, increasing with the magnitude of the numbers. 
