SENSE OF SIGHT. 



587 



affecting our estimates of angular diameter. The explanation of 

 this was first given by Clausius about thirty years ago, . . . but 

 it has not as yet got into the text-books. The circumstance chiefly 

 affecting our estimates of size depends on the angular altitude of 

 the object under consideration. 



" When we pass under an archway or under the limb of a tree 

 we know that we are nearer to the object than we are when we see 

 it under a lower altitude ; at the same time it appears just as large 

 to the average person angularly as it does when we are several feet 

 farther away. We are, in fact, used, all our lives as we walk 

 about, to see objects rapidly shifting their angular positions, yet 

 not appearing as we pass them any larger than they do when we 

 are slightly more distant from them. We thus always uncon- 

 sciously make some compensation in our minds for the real changes 

 in angular size that actually occur. 



" If, now, the limb of the tree that we passed under, instead of 

 really growing angularly smaller at the low altitude than it was 

 when overhead, should remain of the same angular size in all posi- 

 tions, we should say that it looked larger at the low altitude. 

 This is exactly what happens in the case of the heavenly bodies. 

 Unlike all terrestrial objects, they are practically of the same real 

 angular dimensions when on the horizon that they are in the 

 zenith. We involuntarily apply to them the same compensation 

 that we are accustomed to apply to terrestrial objects, and are thus 

 naturally surprised to see that they appear larger at the lower 

 altitude." 



FIG. 362. Formation of solar spectrum. 



Color. When a beam of sunlight passes through a prism it is 

 separated by dispersion into its component colors, forming a solar 

 spectrum (Fig. 362), the red rays being the least refracted, and 

 the violet rays the most. The color depends upon the rapidity of 

 vibration or the length of the waves ; thus the red waves are the 

 longest and the vibrations the least rapid, while the violet are the 

 shortest and the vibrations the most rapid. In the following 



