136 FRAGMENTS OF SCIENCE 



colored covered with pines, for example every peak and 

 ridge is separated from the mountains behind it by a thin 

 blue haze which renders the relations of the mountains 

 as to distance unmistakable. When this haze is regarded 

 through the Nicol perpendicular to the sun's rays, it is 

 in many cases wholly quenched, because the light which 

 it emits in this direction is wholly polarized. When this 

 happens, aerial perspective is abolished, and mountains 

 very differently distant appear to rise in the same vertical 

 plane. Close to the Bel Alp, for instance, is the gorge 

 of the Massa, and beyond the gorge is a high ridge dark- 

 ened by pines. This ridge may be projected upon the 

 dark slopes at the opposite side of the Khone valley, and 

 between both we have the blue haze referred to, throwing 

 the distant mountains far away. But at certain hours of 

 the day the haze may be quenched, and then the Massa 

 ridge and the mountains beyond the Rhone seem almost 

 equally distant from the eye. The one appears, as it 

 were, a vertical continuation of the other. The haze 

 varies with the temperature and humidity of the atmos- 

 phere. At certain times and places it is almost as blue 

 as the sky itself; but to see its color, the attention must 

 be withdrawn from the mountains and from the trees 

 which cover them. In point of fact, the haze is a piece 

 of more or less perfect sky; it is produced in the same 

 manner, and is subject to the same laws, as the firmament 

 itself. We live in the sky, not under it. 



These points were further elucidated by the deport- 

 ment of the selenite plate, with which the readers of the 

 foregoing pages are so well acquainted. On some of 

 the sunny days of August the haze in the valley of the 

 Rhone, as looked at from the Bel Alp, was very remark- 



