268 Professor S. P. Langley [April 17, 



attention to the great practical importance of studying the action of 

 our own terrestrial atmosiDhere on the sun, and the antecedent pro- 

 bability that our 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 colourless 

 atmosphere can have such an effect, and analogous observations which 

 I have carried on for many years, but do not now detail, show that the 

 atmosj^here 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 aj^pears 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 023tical sieve, holding back the blue ; but it may naturally 

 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 recondite 

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

 may gather for ourselves ? " • I think there are, even to our unaided 

 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 colourless 

 and transparent, but it is jDroved not to be so by looking through it 

 edgewise, when the light, by traversing a greater extent, brings out 

 its true colour, which is yellow. Everyone knows this in every-day 

 experience. We shall not get the colour 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 towards the 

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

 true that this air, where we see its real coloui* 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 

 testify 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 Prof. 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 towards us ? I hope it will be under- 

 stood that Prof. Tyndall is in no way responsible for my own 



