waves of varying length and intensity or speed. Not all the rays which make up 
the sun’s radiation reach the earth. If they did happen to do so, even for so short 
a time as one second, it is likely that every living organism on the earth would be 
destroyed, and possibly our planet itself would be burned to ashes. Fortunately 
(or having regard to the mess into which humanity has got itself, perhaps un- 
fortunately), most of these rays are absorbed first by the comparatively narrow 
belt of ozone which lies like a blanket round our atmosphere, second, by the at- 
mosphere itself, and, third, by the clouds, dust, vapour and other impurities which, 
though unseen, crowd the air about us. As the result of these barriers only a 
very limited number of rays outside the visible rays succeed in penetrating 
through to the earth’s surface—and these invisible rays are a fraction of the infra- 
red rays, and a fraction of the ultra-violet rays—modifications of each of which 
are represented by the red and violet rays which comprise the two ends of the 
spectrum or visible light rays as shown in the enlarged section at the bottom of 
the diagram. 
VISIBLE LIGHT GAMMA RAYS FROM 
R-A MATTER 
RADIO WAVES 
COSMIC RAYS|?) 
~~ SOLAR 
<— SHORTER RAYS —> RADIATION 
FASTER 
<- LONGER RAYS —? 
SLowerR 
THE SPECTRUM OF VISIBLE LIGHT 
5 
& 
RAY 
EE 
ne 3 
S 
VISIBLE LIGHT RAYS REACHING THE EARTH AT THE RATE OF 392,70 127 BILLIONS PER SECOND 
We know that the solar energy received by the earth is not constant, in so far 
as it varies in different parts of the world, and at different times of the year, not 
only in its duration, but in its intensity and its quality. Thus at the equator we 
have days which are divided almost equally into darkness and light, while in the 
distant latitudes we find that in summer the days have light for anything up to 
24 hours and in winter they are in darkness for the same time. Then, again, the 
intensity of the sun’s rays is affected by the angle at which they reach the earth 
—if the sun is directly overhead the rays have a much greater intensity than if 
they are striking obliquely at the earth’s surface, for the simple reason that in the 
latter case they have a greater depth of atmosphere, etc., to penetrate, and so more 
of their force is absorbed. Therefore, the intensity of the sun’s rays is greatest. in 
mid-summer from the annual aspect, and at noon during the diurnal period. The 
exact difference of intensity at different periods and times have been calculated 
and demonstrated, but they do not concern us in this paper. 
In the paper on the “Nutrition of Plants” which appears elsewhere in 
this volume, we see, by a process called photo-synthesis, how the leaves of our 
17 
