146 
IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
are dark, the pupil will enlarge to take in a big flood of light and 
a smaller intensity will suffice. 
The question which was at the bottom of this undertaking and 
which really prompted it, was, why a greater intensity of natural 
light is required for the same kind of work than is required of 
artificial light. Since natural lighting is only an extreme case of 
indirect- lighting, it will be evident that our question is answered 
by the above data. It would seem reasonable that such should 
be the case, for with indirect or natural lighting the objects in the 
range of vision are illuminated and the light reflected into the 
eye causes a contraction of the pupil, and hence a greater in- 
tensity is required for clear vision. For the direct artificial light- 
ing most of the light which enters the eye comes from the printed 
page and hence the pupil is larger and clear vision is obtained with 
a lower intensity. 
In the direct lighting system the path of the beam of light 
from the source to the book is short — much shorter than for the 
indirect system. It follows, therefore, that the flux of light on 
the floor near the individual for the direct system is much less 
than on his book. But such is not the case for the indirect sys- 
tem where the path of the beam of light from the source to the 
book is almost the same as from the source to the floor. Thus if 
we had three foot-candles on the book by each system and two 
foot-candles on the floor by the direct system, we should prob- 
ably have 2.8 foot-candles on surrounding objects by the indirect 
system. This latter would cause pupillary contraction and hence 
a greater intensity would be required for clear vision. 
It may be argued that in this experiment the change of current 
through the lamps caused a change in color which served to affect 
the results; but whenever the color was at all abnormal due to 
the small current, the intensity was too low for comfortable read- 
ing and hence did not affect the conclusion here arrived at. 
Again it may be urged that five minutes was too short a time 
in which to form a final opinion on a lighting system, but inas- 
much as these persons had been studying illumination for three 
months, and since an equal period was allowed for each condition 
of illumination, it would seem that the conclusions here, arrived 
at can be considered reliable. 
Physics Laboratory. 
Iowa State College. 
