LIGHT AND HEAT 67 
rays meet the eye, and the plant appears green. Re- 
garded thus, even the colour of the vegetation is an 
expression of its needs. 
Now, in dealing with sunshine and its effect upon vege- 
tation we have to consider two things : 
1. Its intensity. 
2. Its duration. 
The intensity of the sunlight is measured by the amount 
of energy falling upon any space. It varies with the lati- 
tude, diminishing from the Equator to the Poles. During 
the day it increases from dawn to noon, and decreases 
from noon to sunset. The value of a beam of light, so 
far as its energy is concerned, is proportional to the angle 
at which it strikes the earth. When the sun is vertically 
overhead, as it is twice every year within the Tropics, its 
effect is the greatest ; when it is on the horizon its effect 
is least. This fact is well appreciated in photography. 
The chemical salts present in the photographic film are 
affected chiefly by the actinic rays which lie beyond the 
violet end of the visible spectrum. In July the light- 
value for exposure at noon is six times as great as 
at 7 p.m., while in the latitude of London the light- 
value at noon in January is only one-third of that in 
July. 
The duration of light varies with the seasons. At the 
equinoxes, when the sun crosses the Equator (March 2} 
and September 22), day and night are equal throughout 
the world—that is, each is twelve hours long. When the 
sun is overhead at the Tropic of Cancer, on June 24 
(Midsummer Day), we have the longest day (about sixteen 
and a half hours in the latitude of London), and conse- 
quently the shortest night. The sun is, at that time, 
more nearly overhead in the North Temperate Zone than 
at any other, and so has its greatest effect. After mid- 
summer daylight decreases, both in intensity and dura- 
tion, until midwinter, when the days are less than eight 
hours in length. 
This accounts for the great contrast between tropical 
and polar climate. Within the Polar Circle the sun is 
never far from the horizon, while in the Tropics it is 
always nearly overhead. Again, the polar regions in 
winter revolve in continuous shade, and there is no day ; 
