118 KANSAS ACADEMY OF SCIENCE. 



quite certain, in the light of these researches, that we look out into the heavens 

 through a reddish opalescent haze, which greatly modifies the quality of the sun's 

 rays to us, producing an effect not unlike that of Indian summer. 



Professor Langley's study of the absorjition spectrum of the earth's atmosphere 

 has simplified the problem of the color of the sky by excluding a number of hypoth- 

 eses which are incompatible with his results. It necessitates our looking for the 

 blueness of the sky in some action of the atmosphere other than absorption, since 

 his experiments show conclusively that sunlight becomes less and less blue by trans- 

 mission through the air. Much light has been thrown upon the process by which 

 the azure may be formed, by Tyndall's famous experiment, known as the '■'•artificial 

 sky.'''' While studying the decomposition of vapors by sunlight, Tyndall had occa- 

 sion to send a powerful ray of white light through a tube containing a certain or- 

 ganic vapor (the nitrite of amyl). This vapor was broken up by the light, and some 

 of its constituents were precipitated in the form of a fine cloud, which grew denser 

 and denser as the experiment proceeded. This cloud when forming, presented a 

 beautiful blue appearance strikingly suggestive of the open sky, which gave way as 

 the precipitate increased in intensity, to a milky whiteness.^ This incipient cloud 

 Tyndall called his artificial sky, and the belief that it is very closely allied in charac- 

 ter to the true sky, although the substances from which the color arises are of course 

 not the same, is very much strengthened by the evidence of the polariscope. 



One of the most puzzling features of the light reflected from the sky is its polar- 

 ization, which differs from that of the light reflected by water, or indeed by any solid 

 or liquid substance. Now the angle of maximum polarization of the artificial sky 

 was found to be the same as that of the real sky, and the same thing has since been 

 shown to be characteristic of the rays reflected by many other substances when in a 

 state of exceedingly fine subdivision. ^ 



A similar blue has also been produced by Lord Rayleigh, by the precipitation oi 

 sulphur from solutions of the hyposulphite of soda by means of hydrochloric acid. 

 This experiment is so simple, that I will try to reproduce it for your benefit. 



Here, as in Tyndall's experiment, the blue is most marked at the moment when 

 the cloud is beginning to form; and it is speedily supplanted by the milky appear- 

 ance characteristic of opalescent solutions. It is the peculiarity of all the solutions 

 capable of giving the blue cloud to transmit red light and absorb the shorter wave- 

 lengths. When we pass the rays of the lantern through a cell containing the milky 

 liquid obtained in this experiment, the white light upon the screen becomes ruddy, 

 and the effect upon the rays passing through the solution is similar to that which 

 the earth's atmosphere exerts upon the rays of the sun in their journey to the sur- 

 face of our planet. We live, in a word, at the bottom of a vast sea of opalescent 

 material, which transmits the sunbeams to us with their blue and violet components 

 enormously reduced, and which reflects, from the multitudes of exceedingly small 

 particles susjiended in it, light which produces upon the retina the same effect as the 

 light reflected by the "artificial sky" of Tyndall, or of Rayleigh. I shall show you 

 presently that true sky light is almost identical in composition with the light from 

 these artificial skies. 



The experimental evidence concerning the real nature of these colors is exceed- 

 ingly meager. A variety of observations of my own led me some years ago to call 

 in question the generally accepted view that the blue of the sky is of objective char- 

 acter like that transmitted by cobalt glass or produced by reflexion from such pig- 



' Tyndall; Fragments of Science, p. 93; also, Contributions to the Domain of Molecular Physics, 

 p. 425. 



"See Hurch ; " Souje Experiments on Flame ; " Nature, vol. 31, p. 272. 



