170 Mr. J, C. M c Connel on Diffraction-Colours, 



mainly to diffraction, and from the extent of the bright region 

 we can form a rude idea of the size of the particles. The 

 absence of colour will present no difficulty to anyone who has 

 noticed the overpowering brightness of the white central 

 space compared with the surrounding rings of coronee. It is 

 easy to believe that if there be any considerable variety of 

 size, or if, on the other hand, the majority of particles have 

 any shape other than those approximating to spheres or long 

 cylinders, there will be such blurring of colour that nothing 

 but the bright central white space will survive. As to the 

 size of particle, suppose, for example, the white glow, up to 

 the circle where the brightness is about one fourth of that close 

 by the sun, have a radius of 4°. Taking the number appro- 

 priate to the first yellow for filaments and dividing by 4 we 

 obtain 0*0055 mm. We can then assert with some confidence 

 that there are a considerable number of crystals in the cloud 

 at least as thin as 0*005 mm. The estimate is rough, but 

 rough estimates often prove useful. If there is no percep- 

 tible increase of brightness in the cloud near the sun we can 

 assert positively that the vast majority of the particles are in 

 no dimension as thin as 0*1 mm. 



Unusually vivid iridescences were seen in England and Scot- 

 land during the winters 1884-5 and 1885-6, and called forth 

 a number of letters to * Nature.'' The clouds seem to have 

 been of a special type, suspended at a great height in the air, 

 and recognizable even when not coloured *. The bright and 

 varied colours described occurred at distances of over 30° 

 from the sun, so the ice-filaments must have been as fine as 

 0*001 millim. in diameter. Colours similar, though not so 

 bright, were seen nearly opposite the sun. But the region 

 betw r een 50° and 130° from the sun seems to have been 

 colourless, showing that this was not a case of the action of 

 thin plates. 



If cloud-colours are ever formed by thin plates of ice, they 

 may be distinguished by their strong polarization in the plane 

 of the sun. Let us imagine the reflected light to be divided 

 into two parts polarized respectively in and perpendicular to 

 the plane through the sun. Even at 10° from the sun the 

 ratio of these parts is 0'74, at 20° 0'53, at 30° 0'37. In the 

 case of diffracted light the polarization is of the same nature, 

 but not nearly so strong. It probably varies as the square of 

 the cosine of the angle of diffraction. This gives the ratio at 

 10° 0-97, at 20° 0*88, at 30° 0*75. Thus at 10° the polariza- 



* See especially letters by Mr. T. W. Backhouse, Feb. 19, 1885, and 

 Mar. 25, 1886. One cloud, from simultaneous observations at different 

 places, was found to be at least 11 miles high. 



