APPENDIX 341 



less as water or glass ; but if the body arrests some rays and 

 lets others pass through it, it will acquire the colour of the rays 

 it lets through. If we analyse by means of a prism the light 

 reflected by a coloured body, or the light which has passed 

 through a coloured body, it is obvious that we shall no longer 

 obtain an entire spectrum of seven colours, but one from which 

 the absorbed rays will be absent. 



Vegetation presents us with a similar phenomenon. In 

 bright sunlight forests and meadows appear green. It is clear 

 that if the leaf reflects the green colour it must absorb part of 

 the white light which it received. 



Before drawing any inference, however, from this fact, let 

 us investigate more closely the cause of the green colour of the 

 leaves. Whatever green part of a plant we may choose for 



FIG. 82. 



investigation under the microscope, we very soon arrive at the 

 conclusion that in itself it is colourless ; it consists of bubbles 

 called cells, the walls of which are as transparent as glass, and 

 the liquid which fills them as colourless as water. But this 

 liquid contains bodies or grains emerald green in colour. They 

 are generally called chlorophyll granules or chloroplasts. It 

 is to these granules containing chlorophyll that the plant owes 

 its green colour, in much the same way as the blood owes its 

 colour to the red corpuscles which flow in the colourless lymph. 

 Now let us observe what happens to the sunlight when it falls 

 upon the surface of the green leaf ; which rays are going to pass 

 through the leaf and which will be arrested by it ? For this 

 purpose we must let a ray of light pass through the leaf, and 

 then analyse it by means of a prism. When we do so we notice 

 the difference which takes place in the spectrum. The rays 

 absent in the spectrum, those in place of which black spaces 

 are observed, have obviously been arrested by the leaf, have 



