S4 THE POPULAR SCIENCE MONTHLY. 



beam disappears from the parts so cleared, and the space becomes 

 dark. If, therefore, the air were absolutely pure and devoid of matter 

 foreign to it, the azure of the sky would be no longer seen, and the 

 heavens would appear black ; the illumination of objects would be 

 strong and glaring on the one side, and on the other their shadows 

 would be deep, and unrelieved by the diffused light to which we are 

 accustomed. 



Now, setting aside the dust, of which we may hope that there is 

 but little on the downs behind your town, or out to sea in front, there 

 are always minute particles of water floating in the atmosphere. 

 These vary in size from the great rain-drops which fall to earth on a 

 sultry day, through the intermediate forms of mist and of fine, fleecy 

 cloud, to the absolutely invisible minuteness of pure aqueous vapor 

 which is present in the brightest of skies. It is these particles which 

 scatter the solar rays, and suffuse the heavens with light. And it is a 

 curious fact, established by Prof. Tyndall while operating with minute 

 traces of gaseous vapors (which I can only notice in passing, because 

 it belongs only in part to our present subject), that while coarse par- 

 ticles scatter rays of every color equally in other words, scatter 

 white light finer particles scatter fewer rays from the red end of the 

 spectrum, while the finest scatter only those from the blue end. And, 

 in accordance with this law, clouds are white, clear sky is blue. 



But besides this fact, viz., that light scattered laterally from fine 

 particles is blue, the same philosopher perceived that light so scattered 

 is polarized ; and by that observation he again connected the celestial 

 phenomena described above with laboratory experiments. 



By a slight modification of his experiment, due to Prof. Stokes, I 

 hope to make this visible to the audience. It will probably be in your 

 recollection that when polarized light passed through a Nicol, its 

 intensity is unaltered when the Nicol is in one position, but it is de- 

 stroyed when it is in another at right angles to the first. I now pass 

 the beam from the electric lamp through a tube of water containing a 

 few drops of mastic dissolved in alcohol. The mixture so formed holds 

 fine particles of mastic in a state of suspension ; these scatter the light 

 laterally, so as to be visible, I hope, to the entire audience. And if we 

 were to examine with a Nicol this scattered light, we should find the 

 phenomena of polarization. But, better still, we can cause the light 

 to pass through the Nicol before being scattered, and produce the 

 same effect, not only upon the particular part to which our eye is 

 directed, but upon the whole body of scattered light. As the Nicol is 

 turned, the light seen laterally begins to fade ; and when the instru- 

 ment has been turned through a right angle, the only parts remaining 

 visible are those which are reflected from the larger impurities floating 

 in the water independently of the mastic. An effect still more beauti- 

 ful, and at the same time more instructive, can be produced by inter- 

 posing, as was done in the case of reflection, a plate of quartz between 



