66 BRITISH PLANTS 
Light. 
An ordinary beam of white light is a bundle of many- 
coloured rays ; mixed together and falling all at the same 
moment upon the retina, they produce upon the brain an 
effect which is manifested in the consciousness as white 
light. A beam of light may be resolved into its con- 
stituent coloured rays by passing it through a triangular 
prism of glass. Not only are the rays bent or refracted 
in passing through the prism, but each coloured ray 
is bent to a different extent, so that, if the emerging 
beam is caught on a screen, a multicoloured band of light 
is produced which is known as the solar spectrum. This 
spectrum contains all the colours of the rainbow, one 
colour fading into the next imperceptibly. Nevertheless, 
seven primary colours may be recognized following each 
other in order : red, orange, yellow, green, blue, indigo, and 
violet. The red rays are the least refracted by the prism, 
the violet rays the most. This luminous spectrum, how- 
ever, does not contain all the rays which pass through the 
prism. The band is continued at either end by rays 
which, though invisible, produce other effects. The dark 
rays beyond the red end of the visible spectrum, falling 
upon a body, raise its temperature (dark heat-rays) ; those 
beyond the violet end do not perceptibly warm, but they 
act powerfully upon a photographic plate, and so are 
chemically very active (ultra-violet or actinic rays). 
Now, these variously coloured rays are of different 
value in their influence upon plants. The manufacture 
of starch in the green leaf is possible only in the presence 
of light, and it is easy to find out which rays are the most 
useful by growing plants under coloured screens. Under 
a red or yellow screen a plant will be found to be as active 
in making starch as if it were living in ordinary white 
light ; in a blue or violet light little or no starch will be 
formed. We conclude from this that the red and yellow 
rays of the solar spectrum are the most effective in 
photosynthesis, and that the green chlorophyll present 
in the assimilating cells absorbs these colours from the 
beams, and utilizes them in the construction of carbo- 
hydrates. This selection of rays accounts also for the 
colour of green plants; the green pigment absorbs the 
red and yellow rays, but rejects the green ; the rejected 
