288 Mr. J. C. M c Oonnel on Diffract ion- Colours. 



0'013*. An atmosphere is defined to be the mass of air 

 traversed by a line drawn vertically upwards from the level 

 of the sea. The value depends on two series of observations 

 on particularly fine days at South Kensington, when the air- 

 thicknesses were about 1*3 and 3*3 atmospheres. Taking the 

 colour of the sun outside the atmosphere as the point W, the 

 points S 5 , S 10 , S 20 , S 40 on the diagram give the colour of sun- 

 light which has traversed 5, 10, 20, and 40 atmospheres 

 respectively. The first two are yellow inclining to orange, 

 the third a fine orange, and the fourth redder than red lead. 

 To the colour of the fourth, wave-lengths less than 529 con- 

 tribute nothing appreciable ; and even in the third the violet 

 sensation is mainly due to wave-length 663. For an observer 

 at sea-level the first three thicknesses occur when the 

 apparent zenith distance of the sun is 78J°, 85°, and 87|°. 

 For apparent Z.D. 90° the thickness is 35'5 atmospheres f. 

 The additional 4*5 atmospheres can be secured by ascending 

 a height of 330 feet, while from a height of 3000 feet the 

 coloration due to 50 atmospheres can be studied. The same 

 action is exhibited to some extent by clouds near the horizon 

 and by distant snow mountains. For example, the Alps seen 

 from Berne, forty miles away, look yellowish. But here the 

 colour is interfered with by the intervening " blue sky." In 

 other words, the particles, which sift the blue waves out of 

 the light from the snow, send to the observer a by no means 

 negligible quantity of scattered sunlight. 



It is only when the colour of the sun is white that the sky 

 is represented by X~ 4 . If sunlight be represented by S 5 , then 

 skylight will be represented by <r 5 , slightly on the green side 

 of white. The paleness of the sky, when the sun is low, is a 

 familiar phenomenon. Similarly «r 10 , cr 20 , correspond to S 10 , 

 S 2 o- But it is clearly of no consequence whether the shorter 

 wave-lengths are filtered out before or after scattering. So 

 if we could look at the ordinary blue sky through a tube, filled 

 with air, 25 miles long it would appear pale greenish white. 

 In the same way the blue of the sky near the horizon is of 

 poorer quality than near the zenith. When the scattered 

 light either before or after scattering has had to traverse 40 

 atmospheres, its colour reaches the point c 40 on the diagram, 

 i. e. it is really red. This is the red of a sunset sky. It is to 

 be noticed that the form of the curves W, S 5 , S l0 , S 20 , S 40 , and 



* Phil. Trans. 1887. His statements left me in some doubt as to the 

 position of the decimal points, but the evidence of his diagram was 

 decisive. 



t From Foibes's values, quoted by Abney, which allow for refraction 

 and the curvature of the earth. 



