icith reference to Cor once and Iridescent Clouds. 169 



common in England as in the Alps, though fine displays are 

 rare. Common though it be, it has been seen by few. The 

 observer should arm himself with dark spectacles, and, shading 

 his eyes from the direct sunlight with the hand, or preferably 

 with some distant object, such as a building, carefully examine 

 any clouds within 10° of the sun, and he will seldom fail to 

 see colour. A curious illustration of this fact is that irides- 

 cences are so frequently seen when the brightness of the sun 

 is reduced by an eclipse, and recorded as remarkable pheno- 

 mena. To meteorologists these colours should be especially 

 interesting, as manifesting the actual size of the cloud par- 

 ticles *. The following table will be found convenient. To 

 calculate the diameter in millimetres of the average filament 

 in an ice-cloud or of the average drop in a water-cloud, divide 

 the number in the first or second column respectively by the 

 angular distance of the colour from the centre of the sun 

 expressed in degrees. 





Filaments. 



Drops. 



First yellow -. 



0022 

 0-028 

 0-031 

 0035 

 0-045 

 0-051 

 0057 

 0-062 

 076 

 0-087 

 0-106 

 0-124 



0028 

 0-035 

 0038 

 0-042 

 0052 

 0-058 

 0-065 

 0-069 

 0-083 

 0095 

 0-113 

 0131 



} , red 





Second blue 



,, green 







, , purple 



Third green 



red 



Fourth green 



„ red 





A convenient and, in general, sufficiently accurate method 

 of measuring the angle is to find the corresponding length on 

 a pencil held at the full stretch of the two arms. The distance 

 from the eye to the pencil varies only slightly with the posi- 

 tion of the body and the altitude of the sun, and can be 

 measured at leisure. 



Even when there is no trace of colour we generally see in 

 moderately translucent clouds a great increase of brightness 

 near the sun. With water-drops this might be produced by 

 ordinary refraction, but in frozen clouds it must be attributed 



* Fifty years ago Kaemtz used Fraunhofer's results to determine the 

 diameter of the vesicles, as he supposed them to be, of fog- (Kaemtz, 

 ' Meteorology,' translated by Walker, p. 111). But the method seems to 

 have been since neglected. 



