Effects of Uniformity and Contrast on Light. 
129 
for weeks. Temporary blindness is also due to sudden excess light 
absorption. 
Daylight, the best diffused of all, causes less injury by overheating 
than artificial light. In the latter the percentage of visible radiation is 
small, the ultra-red portion containing most. But the light obtained from 
modern illuminants is caused by temperature radiation — hence the amount 
of power which enters the eye from artificial lights is much greater than 
from an equivalent daylight illumination. The latter is based on 
luminescence or selective radiation, possesses therefore a higher per- 
centage of visible radiation, and is less harmful. It is, of course, 
assumed that direct looking into the sun, or into reflected sunlight, is 
avoided. 
The type of radiation is also of great importance. The effects caused 
begin with the blue radiation and continue up to the X-rays, the greatest 
effect being given by the extreme of the ultra-violet. The ultra-violet 
radiation extends far beyond the visible range. The visible part consists 
of long waves which are harmless as long as their intensity is limited. 
In artificial- and day-light they are moderately represented, and cause 
therefore little harm. The invisible portion may be divided into tw^o 
sections — waves with a moderate frequency and such with high fre- 
quencies. The latter need hardly be considered here. They occur in 
a Geisler tube, but as the discharges take place within a glass tube the 
high-frequency waves are completely absorbed. They are also set up in 
a low-temperature mercury vapour arc, where they are again absorbed 
by the glass. Quartz does, however, not do so, and such lamps should 
therefore be avoided. In the high- temperature mercury vapour quartz 
lamp the high frequency ultra-violet rays do not occur. 
Of greater importance are the rays of medium frequency, since they 
are set up in most modern illuminants. They are largely damped by 
clear glass. Consequently no one should look into a naked electric arc 
with the bare eye, as these rays are highly destructive. Moreover, the 
injury caused in this way is almost incurable. It is not external, but 
internal, and the eye loses the ability to keep anything in focus for 
any length of time. 
Seeing that the ultra-violet rays are damaging to the eyesight we 
should guard against them. This can be done by the employment of 
proper globes. A mixture of red and green glass, giving a yellowish 
shade, will completely absorb the ultra-violet rays. Moreover, we should 
diffuse the light as much as possible, because by employing reflected light 
the ultra-violet rays are absorbed. It does not materially reduce the light, 
since the damaging ultra-violet rays take no part in giving light. As 
regards electric incandescent lighting, only such shades should be used 
which diffuse the light properly and obscure the lamp bulb. An advance 
