SUNLIGHT AND THE EARTH'S ATMOSPHERE. 649 



atmosphere between the lamp and the screen absorb, it is not enough 

 to show. 



Our spectrum, as it appears before absorption, might be compared 

 to an army divided into numerous brigades, each wearing a distinct 

 uniform, one red, one green, one blue, so that all the colors are repre- 

 sented each by its own body. If, to represent the light absorbed as it 

 progresses, we supposed that the army advances under a fire which 

 thins its numbers, we should have to consider that (to give the case of 

 nature) this destructive fire was directed chiefly against those divisions 

 which were dressed in blue, or allied colors, so that the army was 

 thinned out unequally, many men in blue being killed off for one in 

 red, and that by the time it has advanced a certain distance under fire 

 the proportion of the men in each brigade has been altered, the red 

 being comparatively unhurt. Almost all absorption is thus selective 

 in its action, and often in an astonishing degree, killing off, so to speak, 

 certain rays in preference to others, as though by an intelligent choice, 

 and destroying most, not only of certain divisions (to continue our illus- 

 tration), but even picking out certain files in each company. Every 

 ray, then, has its own individuality, and on this I can not too strongly 

 insist ; for just as two men retain their personalities under the same 

 red uniform, and one may fall and the other survive, though they 

 touch shoulders in the ranks, so in the spectrum certain parts will be 

 blotted out by absorption, while others next to them may escape. 



To illustrate this selective absorption, I put a piece of didymium 

 glass in the path of the ray. It will, of course, absorb some of the 

 light, but, instead of dimming the whole spectrum, we might almost 

 say it has arbitrarily chosen to select one narrow part for action, in 

 this particular case choosing a narrow file near the orange, and letting 

 all the rest go unharmed. In this arbitrary way our atmosphere 

 operates, but in a far more complex manner, taking out a narrow file 

 here and another there, in hundreds of places, all through the spec- 

 trum, but on the whole much the most in the blue, the Fraunhofer 

 lines being merely part of the evidence of this wonderful quasi-mtelli- 

 gent action which bears the name of selective absorption. 



Before we leave this spectrum, let us recall one most important 

 matter. We know that here beyond the red is solar energy in the 

 form of heat which we can not see, but not on that account any less 

 important. More than half the whole power of the sun is here invisi- 

 ble, and, if we are to study completely the action of our atmosphere, 

 we shall have to pay great attention to this part, and find out some 

 way of determining the loss in it, which will be difficult, for the ultra- 

 red end is not only invisible, but compressed, the red end being shut 

 up like the closed pages of a book, as you may notice by comparing 

 the narrowness of the red with the width of the blue. 



Now, refraction by a prism is not the only way of forming a spec- 

 trum. Nature furnishes us color not only from the rainbow, but from 



