128 Transactions of the Royal Society of South Africa. 
plays an important part. In looking at a light before a dark screen the 
eye endeavours to do two things at once — the pupil tries to open wide to 
take in the dark background and to close up for the intrinsic brilliancy 
of the source. This phenomenon of double intentions really constitutes 
glare. The human eye tolerates enormous differences of diffused light, 
which we get from the light before a white screen, but it objects to high 
intrinsic brilliancy when it is accompanied by great contrasts such as a 
dark background. 
A few remarks on the physiological effects of radiation may not be out 
of place. 
The illuminants of old were of low power and great wave-length, the 
light being very red. With the advent of the gas mantle, the metal fila- 
ment electric lamp, and the arc lamp, the light has become whiter and 
of much shorter wave-length. The lights of old were mostly too poor 
to enable people to work with artificial light ; and this not being done, 
people suffered less from weak eyes than the present generation. Now 
the average person has to work three to four hours daily with artificial 
illumination, and it is therefore of the greatest importance that the work 
can be carried out with the least injury to the eyes. Many people have 
complained to me that electric light gives them headaches after an hour's 
working. They put it chiefly down to the high percentage of ultra-violet 
rays contained in the electric light. And yet daylight contains an even 
higher percentage of these rays, but it does not affect these persons."' 
Without the slightest doubt modern illuminants have caused harm to 
the eyesight of many people. This is entirely due to improper illuminating 
engineering, whereby the destructive rays were given full scope, either 
by diffusing the light too little and causing simultaneous great contrasts 
or by directing the light wrongly. Even a small light — an ordinary wax 
candle — causes glare if it is directed into the eye under great contrasts. 
The eye attempts to do two things simultaneously. When contrasts are 
avoided the eye can protect itself. Moderate excesses are shut off by con- 
traction of the pupil, sudden abnormal quantities of light by closing of the 
eyelids. Light has two different effects on the eye — the one is caused by 
the quantity, the other by the type of radiation. 
Eadiation is power. The light which enters the eye is absorbed and 
converted into heat. If the rate of conversion becomes too great the 
heating becomes excessive and the eyes give pain. If the " overheating " 
continues the eyes become inflamed and tears run. Was the time of 
overheating comparatively short, a few hours will see full recovery ; 
otherwise, with continued excess absorption, the damage done may last 
* I wish to point out here that the sunlight has an effect of this nature on me. By 
wearing, however, ordinary glasses (for shortsightedness) headaches practically disappear. 
An explanation will be given later on. 
