132 
Transactions of tJie Boyal Society of South Africa. 
inverted lamps are not often er employed. From a series of experiments I 
come, however, to the conclusion that the waste is more apparent than 
real. For the purpose of carrying out the tests I had six tantalum lamps 
fixed to a wooden frame in such a manner that they could easily be pro- 
vided with all kinds of shades and also be inverted. The lamps, or rather 
the illumination, were then tested by means of an illumination photometer, 
(1) in a room with jet-black walls and ceiling, and afterwards in an office 
of slightly larger size with a white ceiling and light yellow walls. The 
results are shown in Fig. 1. We notice that the illumination increases as 
we pass from the dark to the light room, the difference being shown by the 
lengths of the ordinates lying between the curves (1) and (3). I expected 
a much greater increase, but as three sets of experiments yielded the same 
result a mistake seems improbable. The six lamps were then provided 
with holophane shades, whereby the illumination in the test plane (hori- 
zontal, 1 metre above floor) was further increased. Finally, the room was 
illuminated with 4-40-watt Osram lamps within long fancy shades, point- 
ing upwards under an angle of 45°, 335 centimetres above the floor. The 
result was a uniform illumination of ]3 candle-metres. 
Tests were made in reading text-book print by means of the various 
illuminations. 
In the dark room, an illumination of 35 to 40 candle-metres seemed 
essential for comfort. It was, however, necessary to keep the eye on the 
book, as roaming about caused glare. In the light room, w^here contrast 
was almost absent even with the lamps hanging dowmwards, 25 to 30 
candle-metres seemed sufficient, and roaming about the room with the eyes 
did not cause glare as long as one did not look directly into the filament. 
When the lamps were inverted, and a perfectly uniform illumination 
resulted, comfort was achieved even with such a low illumination as 
13 candle-metres. Twenty candle-metres gave perfect results. More- 
over, glare was entirely absent, as the bulbs were out of the line of vision. 
We see, therefore, that although by indirect lighting we waste light, the 
resulting uniformity requires less. Moreover, the eye may easily adapt 
itself to great differences in illumination if the latter be uniform. I have 
seen it frequently stated lately that the human eye objects to an artificial 
illumination of more than 80 to 100 candle-metres. I do not agree with 
this. The eye, according to my own experience, will stand an enormous 
illumination if the latter is thoroughly diffused and contrasts are avoided. 
If we could get daylight diffusion, and prevent contrast, the eye would 
stand thousands of candle-metres without harm. 
Let us try to form a reason for the reduction in light required with an 
increase in uniformity, or rather with the avoidance of contrasts.''' 
* Curve 3 in Fig. 1, which was taken in the light room, does not show such a very 
great increase in the uniformity of the light. Contrast had, however, almost disappeared. 
