the Colour of the Sky. 431 



comparison, are apt to realize ; and yet the two kinds of 

 light are identical in composition, varying from ordinary white 

 daylight only in intensity. 



In general, white light will seem bluer and bluer as its 

 intensity diminishes, and this law will apply to the skies : 

 as the light they reflect becomes fainter and fainter, they will 

 increase in blueness, even though the light by the process of 

 reflexion suffer no change in composition. 



Nature offers some beautiful demonstrations of this fact*. 

 Study the clouds at sunset at a time when the upper sky is full 

 of fleecy cirri. Such clouds in a noonday light are of the 

 purest white. Watch them now as the beams of the setting 

 sun strike them horizontally. The western edges are of a 

 brilliant white ; the eastern portions are in deep shadow. 

 Compare the lines of the various parts of the cloud — not for- 

 getting that (different as they look), since the structure of the 

 cloud does not vary, and since the light reflected from it is 

 everywhere common daylight, the whole cloud should be, 

 " objectively " speaking, of one colour, white. The entire 

 shaded portion is nevertheless unmistakably blue ; and the 

 deeper the shadow, the more nearly it will approach the colour 

 of the sky. On the sun-lit side the thinnest shreds of mist 

 stand out bright and sharp, like snow-flakes against the blue 

 void behind them. In the shadow, cloud and sky merge into 

 one another, so nearly identical are their hues. 



This phenomenon and that of the blueness of the sky are, 

 then, cases of the effect of intensity of the light upon its hue 

 (subjectively considered). Indeed they are the converse of 

 the well-known change of blue light into white (see p. 430). 

 A ray affecting at ordinary intensities the "violet" nerve- 

 termini chiefly becomes capable, by a peculiarity of the eye, 

 of giving, when bright enough, the red and green impressions 

 also, and thus of appearing white. The same peculiarity of 

 the eye makes a ray which is white when of ordinary bright- 

 ness, seem blue as it becomes faint. In the latter case the 



* Helmholtz gives as the reason why these effects come so rarely into 

 notice, that the daylight which we unconsciously adopt as our standard 

 changes in the same manner as the light we are observing, so that the 

 effect is neutralized. " Dass das Sonnenlicht, welches wir bei Tage als 

 das normale Weiss betrachten, selbst, bei verschiedener Lichtstarke, in 



ahnlicher Weise seine Farbe andern muss wie die anderen " The 



appearance of blue light in deep narrow holes in snow-drifts is probably 

 due to the same cause as the blue colour of the sky. As soon as such a 

 hole is so deep that the interior is dark, the lower part is seen to be per- 

 vaded by a beautiful blue light. The snow affords a colourless screen, 

 which, if the drift be large, is only penetrated by the merest traces of 

 light. These, by virtue of their very small intensity, appear blue. 



