﻿454 M. G. Quincke on the Capillary Phenomena 



as representing with sufficient approximation the entire series. 

 Indeed the residual blue appears to be rather capricious in its ap- 

 pearance, and to depend on conditions not yet fully known. I 

 may mention that I have not been able to detect any unusually 

 intense coloration in that part of the light from the sky which 

 vibrates in a plane passing through the sun. This is the more 

 remarkable, because it might be supposed that a part at least is 

 light which has twice undergone diffraction, in which case the 

 intensity would vary as \~ 8 if otherwise undisturbed. But we 

 must not forget that, of the indirect light illuminating the higher 

 strata of our atmosphere, a very considerable fraction must come 

 from the earth itself; and this certainly is coloured any thing but 

 blue. It would be interesting to observe whether the residual 

 light from parts of the sky 90° distant from the sun is in any way 

 dependent on the character of the earth's surface — whether, for 

 example, it is the same as usual over water or when the ground 

 is covered with snow. I presume that with the precipitated 

 clouds there is no question of light diffracted more than once. 



Theory would lead us to anticipate that the optical density of 

 the particles of foreign matter may have a large influence on the 

 development of the residual blue. If the particles and the me- 

 dium in which they are suspended have nearly the same refran- 

 gibility, the light emitted parallel to the original vibrations may 

 be expected to be very feeble, not only absolutely, but in compa- 

 rison with that emitted in other directions. Professor TyndalPs 

 method of precipitating organic vapours (some of which may 

 have a high optical density) in air is then more favourable than 

 the suspension of mastic or other moderately dense solids in 

 water, as used by Briicke and other physicists. 



I take this opportunity of referring to the observations of 

 Roscoe on the photographic power of skylight, with which I 

 have only lately become acquainted. The comparison of photo- 

 graphic with luminous intensities is well adapted to exhibit dif- 

 ferences of quality related in a simple manner to the wave-length. 

 The very small chemical action of the direct solar rays, as com- 

 pared with what might have been expected from their intense 

 action on the retina, is a striking verification of the theoretical 

 results developed in the February Number of this Magazine. 



LIX. On the Capillary Phenomena of the common Surface of two 



Liquids, By G. Quincke. 



[Continued from p. 390.] 



V. On the spreading of liquids on other liquids. 



24. T¥ three common surfaces of three liquids intersect in a 



-»- (curved) line, then three forces operate on a particle of 



the substance^ P, in the line of intersection, which lie in the per- 



