ATMOSPHERIC POLARIZATION. ]^43 



tlie rotatory power depouds, disappears in tlic same manner as it is known to do 

 in qnartz on fusion. It is impossible within the limits to which we are here 

 confined to pursue this interesting subject further. 



Mr. Arago early made the discovery that the light which comes to us from 

 the atmosphere is polarized. Observations made in the vertical plane passing 

 through the sun show sensible polarization in that plane up to about 150° from 

 the luminary — a point which can only be observed, therefore, when the sun is 

 low. The polarization at this point becomes zero, and it is hence known as 

 Arago's vieutral point. Below this point down to the horizon, polarization is 

 found in a horizontal plane. Mr. Babinet discovered a second neutral point 17° 

 above the sun, and Dr. Brews-ter a third, 8^ 30' below. Neither of these is 

 easy of observation, in consequence of the proximity of the sun himself and his 

 great light. Between them the light is probably polarized horizontally; but the 

 fact, for the reason just mentioned, has not been verified. The plane of polar- 

 ization in the vertical between the neutral points of Ai-ago and Babinet is easily 

 accounted for by ascribing the polarization itself to direct reflection of the sun's 

 rays from the molecules of the atmosphere. The polarization in a horizontal plane 

 below Arago's point is a less simple phenomenon. It is believed, however, to 

 be occasioned by rays which have undergone two reflections from the atmos- 

 pheric molecules. Of the rays of this class those which will come most efiPectively 

 to the eye of the observer will be such as are reflected in the lower parts of the 

 atmosphere in planes nearly parallel to the horizon. These will, of course, be 

 polarized in planes nearly horizontal, and if in force sufficient to overcome the 

 light polarized vertically, will produce a resultant in their own direction. At 

 an altitude at which the two opposite polarizations balance each other, vv'ill be 

 found a neutral point, and this is the point of Arago. 



Regarding atmospheric reflection of the sun's rays as the cause of atmospheric 

 polarization, it Avill follow that every plane passing through the sun (in the 

 superior portions of the atmosphere at least) must be a plane of polarization. 

 This will therefore be true of the hour-circle or meridian in which the sun 

 happens at any time to be. And as all hour-circles pass through the pole of the 

 heavens, it results that a delicate polariscope, directed toward the pole, may 

 follow the horary motion of this plane. Such a polariscope, furnished with a 

 dial and index,' becomes a chronometer. This is the principle of an elegant 

 little instrument invented by Wheatstone, called the polar clock. When accu- 

 rately adjusted, it will indicate, in the hands of a practiced observer, the 

 apparent solar time within a very feAv minutes. It will operate even when the 

 sky is overcast with clouds, provided there be an unobscured spot at the pole, 

 through which the blue sky may be seen. 



In the foregoing very succinct outline of the history of optical discovery^ the 

 object kept in view has been to present simply facts, without entering into any 

 discussion of the physical causes to which they are to be attributed. It is now 

 proposed to consider in what manner these facts may be most satisfactorily 

 explained. 



THEORIES OF LIGHT. 



Two theories have been maintained in regard to the nature of light, either of 

 which is supported by the authority of very illustrious names. According to 

 the first of these, light is a material emanation thrown off by the luminous body, 

 and its particles constantly traverse and fill the entire illuminated space, so long 

 as the source continues unexhausted. According to the second, there is no 

 transfer of matter from the source of light to the surrounding region, but there 

 is a transfer o^ force through the medium of an elastic fluid which fills all space, 

 and whose molecules in contact with the luminous body, being disturbed by that 

 body, transmit the disturbance to those more remote, by means of undulations 



