192 APPENDIX TO MEMOIR OF PELTIER. 



larly, for sucli rays conduct themselves like the rays of natural light. M. Arago 

 has remarked, moreover, that this point does not always correspond to the ver- 

 tical plane passing by the sun, but that it is sometimes found diverted to the 

 right or to the left, when the reflection is altered in one of the reflecting sectors, 

 whether this alteration proceeds from the presence of clouds, or from that of 

 difliised vapors, or from the neighborhood of a mountain, &c. 



We will not enter into further details on this subject; it may suffice to direct 

 the readei-'s attention to the original researches of M. Arago, as well as to those 

 of MM. Quetelet and Delezenne.* It may be stated, however, that a second 

 neutral point has been found by M. Babinet, about 30° above the setting sun, 

 and a third by Mr. Brewster, below the setting sun; but these two neutral points 

 are rather difficult to be observed. All these neutral points, it may be added, 

 exist only in the meridian or in the great circles but slightly remote from it. 



We have supposed that the pile was placed in the plane of the meridian, and 

 that it presented itself perpendicularly to the incident ray. If during the move- 

 ment communicated to the eye-glass along the meridian, we incline the pile on 

 the axis of the himinous pencil, the intensity of the colors will be seen to dimin- 

 ish ; in a word, the pile then depolarizes the atmospheric ray. It depolarizes it, 

 however, unequally, according to the direction that is given to its inclination; 

 for if it is inclined towards the sun it depolarizes rapidly and completely, while 

 if it is inclined in the other direction, towards the opposite pole, it depolarizes 

 much less and often very little. 



If now tlie tube of the eye-glass be turned in its collar, so that the plane of 

 the pile shall be perpendicular to the meridian, and if, in this new position the 

 pile be inclined on the incident ray, the intensity of the tints is augmented, 

 instead of diminishing as in the previous case. Thus, in the first position, that 

 of the pile in the plane of the optical meridian, the pile in inclining depolarized 

 the rays of the atmosphere ; in the second, on the contrary, it adds new polar- 

 ized rays to those which already existed. 



Instead of placing the instrument in the plane of the meridian, the observer 

 may place it in the plane of the equator; suppose, then, the objective tube of 

 the polarimeter in this latter plane, and that the jiile also be in the same 

 plane, and thus presents itself perpendicularly to the incident rays ; if now we 

 direct the instrument in succession to all jioints of the equator, from the maxi- 

 mum point, which has its place on the meridian at the intersection of these two 

 great circles, to the horizon, we shall find the extraordinary image colored blue ; 

 moreover that it preserves the same intensity in the whole line of the eqi;atorial 

 circle; only in approaching the horizon, the tint becomes a little weakened, 

 through the vapors diff"used in the strata of air very near the surface of the globe. 

 We have supposed the pile in the plane of the eijuator and perpendicular to the 

 incident rays; if we incline it on those rays the intensity of the colors is aug- 

 mented; if, on the contrary, we place it perpendicularly to that plane its inclina- 

 tion depolarizes the atmospheric rays and renders the images colorless. 



Thus in the two great rectangular circles which w^e have been considering, 

 one forming the meridian of the optical sphere and the other its equator, the 

 extraordinary ray is blue, the ordinary ray has the complementary tint, orange 

 yellow ; the pile adds to the atmospheric polarity when it is parallel to the 

 equator, while, on the contrary, it depolarizes when perpendicular to it. As to 

 the horizon, if Ave examine it at the moment of the rising or setting of the sun, 

 it will be found that the coloration is null for the rays which proceed directly 

 from that luminary; that it increases up to about 90°; that it then diminishes till 

 about 180°, where it is at its minimum ; that it recovers anew till about 270°, 

 where is found a second maximum, to again disappear when it falls within the 

 too direct rays of the sun. 



* See the Correspondance MatMmatique de M. Quetelet, t. 1, pp. 275 and 338. 



