70 Captain W. de W. Ahney [Feb. 25, 



and you will see that it is not very blue, and, in fact, not bluer per- 

 ceptibly than that we have at the Riffel, the colour of the sunlight at 

 which place I show in a similar way. I have also j^repared some 

 screens to show you the value of sunlight after passing through five 

 and ten atmospheres. On an ordinary clear day you will see what a 

 yellowness there is in the colour. It seems that after a certain 

 amount of blue is present in white light the addition of more makes 

 but little difference in the tint. But these last patches show that the 

 light which passes through the atmosphere when it is feebly charged 

 with particles does not induce the red of the sun as seen through 

 a fog. It only requires more suspended particles in any thickness 

 to induce it. 



In observations made at the Riffel, and at 14,000 feet, I have 

 found that it is possible to see far into the ultra-violet, and to dis- 

 tinguish and measure lines in the sun's spectrum which can ordina- 

 rily only be seen by the aid of a fluorescent eye-piece or by means 

 of photography. Circumstantial evidence tends to show that the 

 burning of the skin, which always takes place in these high altitudes 

 in sunlight, is due to the great increase in the ultra-violet rays. It 

 may be remarked that the same kind of burning is effected by the 

 electric arc light, which is known to be very rich in these rays. 



Again, to use a homely phrase, " You cannot eat your cake and 

 have it." You cannot have a large quantity of blue rays present in 

 your direct sunlight, and have a luminous blue sky. The latter must 

 always be light scattered from the former. Now, in the high Alps 

 you have, on a clear day, a deep blue-black sky, very different indeed 

 from the blue sky of Italy or of England ; and as it is the sky which 

 is the chief agent in lighting up the shadows, not only in those 

 regions do we have dark shadows on account of no intervening — what 

 I will call — mist, but because the sky itself is so little luminous. In 

 an artistic point of view this is important. The warmth of an English 

 landscape in sunlight is due to the highest lights being yellowish, 

 and to the shadows being bluish from the sky-light illuminating them. 

 In the high Alps the high lights are colder, being bluer, and the 

 shadows are dark, and chiefly illuminated by reflected direct sunlight. 

 Those who have travelled abroad will know what the efi'ect is. A 

 painting in the Alps, at any high elevation, is rarely pleasing, 

 although it may be true to Nature. It looks cold, and somewhat 

 harsh and blue. 



In London we are often favoured with easterly winds, and these, 

 unpleasant in other ways, are also destructive of that portion of the 

 sunlight which is the most chemically active on living organisms. 

 The sunlight composition of a July day may, by the prevalence of an 

 easterly wind, be reduced to that of a November day, as I have proved 

 by actual measurement. In this case it is not the water particles 

 which act as scatterers, but the carbon particles from the smoke. 



Knowing, then, the cause of the change in the colour of sunlight, 

 we can make an artificial sunset, in which we have an imitation light 



