646 THE POPULAR SCIENCE MONTHLY. 



own air was also and independently making the really blue sun into 

 an apparently white one. We actually know, then, beyond conjecture, 

 by a comparison of the sun's atmosphere, where it is thickest and 

 where it is thinnest, that an apparently colorless atmosphere can hare 

 such an effect, and analogous observations which I have carried on for 

 many years, but do not now detail, show that the atmosphere of our 

 own planet, this seemingly clear air in which we exist like creatures at 

 the bottom of the sea, does do so. 



We look up through our own air as through something so limpid 

 in its purity that it appears scarcely matter at all, and we are apt to 

 forget the enormous mass of what seems of such lightness, but which 

 really presses with nearly a ton to each square foot, so that the weight 

 of all the buildings in this great city, for instance, is less than that of 

 the air above them. 



I hope to shortly describe the method of proof that it too has been 

 acting like an optical sieve, holding back the blue ; but it may natu- 

 Tally be asked, " Can our senses have so entirely deceived us that they 

 give no hint of this truth, if it be one ? is the appeal wholly to recon- 

 dite scientific methods, and are there no indications, at least, which 

 we may gather for ourselves ? " I think there are, even to our un- 

 aided eyes, indications that the seemingly transparent air really acts 

 as an orange medium, and keeps the blue light back in the upper sky. 



If I hold this piece of glass before my eyes, it seems colorless and 

 transparent, but it is proved not to be so by looking through it edge- 

 wise, when the light, by traversing a greater extent, brings out its 

 true color, which is yellow. Every one knows this in every-day expe- 

 rience. We shall not get the color of the ocean by looking at it in a 

 wine-glass, but by gazing through a great depth of it ; and so it 

 is with the air. If we look directly up, we look through where it is 

 thinnest ; but, if we look horizontally through it toward the horizon, 

 through great thicknesses, as at sunrise or sunset, is it not true that 

 this air, where we see its real color most plainly, makes the sun look 

 very plainly yellow or orange ? 



We not only see here, in humid English skies, the " orange sunset 

 waning slow," but most of us in these days of travel can perfectly tes- 

 tify that the clearest heavens the earth affords, the rosy tint on the 

 snows of Mont Blanc, forerunning the dawn, or the warm glow of the 

 sun as he sets in Egyptian skies, show this most clearly show that 

 the atmosphere holds back the blue rays by preference, and lets the 

 orange through. 



If, next, we ask, " What has become of the blue that it has stopped?" 

 does not that very blue of the midday sky relate the rest of the story 

 that blue which Professor Tyndall has told us is due to the presence 

 of innumerable fine particles in the air, which act selectively on the 

 solar waves, diffusing the blue light toward us ? I hope it will be un- 

 derstood that Professor Tyndall is in no way responsible for my own 



