130 Transactions of the Royal Society of South Africa. 
in this direction is shown in the prismatic glass shade, the so-called holo- 
phane shade, which diffuses the light well. It is, however, not deep 
enough to obscure the lamp bulb, so that the light should be out of the 
line of vision. It would even be an improvement to make the bulb itself 
of red-green glass. 
When light is properly diffused the eye is not called upon to do two 
things simultaneously, i.e., adapt itself to dark shadows and great intrinsic 
brilliancy at the same time. In wireless telegraphy, electric furnace work, 
electric welding, where naked arcs are employed, the operators should be 
protected by suitable glasses in order to prevent overheating and injury 
due to ultra-violet rays of medium frequency. 
In taking once a number of students over some steel works at Sheffield 
we were shown, amongst other things, electric welding apparatus. As 
the number of protecting glasses was limited I looked for a few seconds 
with my naked eye at the arc, formed between the pieces to be welded 
together. The result was such injury to the eyes that they have not 
even fully recovered to-day. This was more than six years ago. I also 
remember w^aking up the following night with a maddeniog pain in my 
eyes, which felt as if they were full of pins and needles. It lasted for 
about an hour. Had I even worn ordinary clear glasses the effect would 
not have been so bad. In wearing glasses care should be taken that 
ultra-violet rays cannot enter the eye above or below the glasses. 
Most people in this country will have experienced excessive glare at 
the seaside on sunny days. The illumination exceeds at noon often a 
hundred thousand candle-metres. The sunlight is largely reflected by the 
white sand, so that the intrinsic brilliancy is abnormally high. Moreover, 
the darkish sea provides a dark background, and glare is the result. 
Similar effects are shown in the Arctics, resulting finally in snow blind- 
ness. The latter may be avoided by the wearing of red and green glasses, 
as was proved by Sir Ernest Shackelton on his dash to the South Pole. 
Through glasses of this nature one may even look directly into the sun 
for minutes without harm. Headaches caused at the back of the eye 
by excess solar radiation or by artificial lights may also be prevented by 
the wearing of red and green glasses. 
My remarks will have made the following points clear : — 
(1) The illumination should be as uniform as possible. 
(2) Direct light should be avoided. 
(3) Naked lights should not be employed without adequate protection. 
As regards (1), it is not difficult to produce a uniform illumination 
either during the day or at night. 
Daylight can hardly be made excessive if glare is avoided. Nature 
has provided the eye with everything for its protection as long as glare 
does not occur. There is, therefore, from this standpoint nothing to be 
