52 INTRODUCTION TO GENERAL PHYSIOLOGY 



an appropriate system. It will be perfectly obvious that none of 

 these rays can produce any change in bodies on which they fall 

 unless they are absorbed. To produce change requires energy, 

 and if the energy of the light is as great after passing through a 

 body as before it impinges upon it, no energy has been given 

 up to that body, and no effect produced in it. All raj's from the 

 sun can be converted into heat when absorbed, and their energy 

 measured in this way. But, as we have seen, if we want the most 

 efficient conversion of their energy into other forms, such as 

 chemical energy, it must take place without previously passing 

 through the stage of heat. This is ensured by the aid of certain 

 coloured substances which absorb the energy of light, and enable 

 it to effect chemical changes directly. These substances are 

 sometimes called "optical sensitizers," because they make it 

 possible for a chemical system to absorb light of a wave-length 

 which it would otherwise be unable to do. A familiar instance 

 is the dye with which " red-sensitive " photographic plates are 

 stained. 



If we take a coloured solution, say one of a green tint, and look 

 through it at a white surface, we realise that the light which reaches 

 the eye must be that which has not been absorbed. A green 

 solution absorbs the light of both ends of the spectrum, leaving the 

 green part in the middle. If we next examine a solution of 

 chlorophyll with a spectroscope (E., p. 186), we find that there is a 

 particular region in the red in which the light is greatly absorbed, 

 showing a dark band in dilutions such that very little absorption is 

 shown elsewhere. A spectroscope is an instrument which sorts out 

 the mixed wave-lengths of white light in series, according to their 

 wave-length. This it does by means of a prism, or other device, 

 which deflects the rays from their straight course in proportion to 

 their wave-length on account of the fact that in passing through 

 the prism the red rays are turned aside less than the violet rays. 

 They are deflected less because the rate of propagation of light 

 waves in a dense medium like glass is lower than in air, and that 

 of rays of short wave-length is affected more than that of the 

 longer ones. So that when a wave-front strikes obliquely, more 

 effect is produced on the shorter waves. 



The reason why a substance absorbs rays of a particular wave- 

 length is because the rate of vibration of certain of its molecular 

 constituents coincides with that of the light absorbed. The energy 

 of the light is thus transferred to the absorbing substance by what 

 is known as " resonance? This may be understood by taking a 

 pendulum at rest and giving a series of very slight blows. The 

 first of these will produce a very small movement of the pendulum, 

 which will swing back beyond its resting position and then return 



