of Iridescence in Clouds. 93 



figure occurred along with the last-described genus [Hexagonal pyramids]. 

 Of which kinds, principally, a quantity of snow, three or four inches in 

 depth, once fell on the deck of the ship in which I sailed, in the course 

 of a few hours. The temperature when this kind of crystal fell was in 

 one instance 22°, and in the other 20° " [Fahrenheit]. 



In four engraved plates Captain Scoresby delineates ninety- 

 six different forms, magnified from three to seven times. And 

 he attaches a letter to most of them referring to an annexed 

 table, in which the state of the atmosphere and weather, when 

 each form was observed, stand recorded. In what he further 

 adds to his record, he says : — 



" Many instances, it may be observed, occur of mutilated and irregular 

 specimens, some wanting two or three radii, and others having radii of 

 different shapes. But in low temperatures the greatest proportion of 

 crystals that fall are probably perfect geometrical figures." 



The foregoing are the parts of Captain Scoresby's record 

 of his invaluable observations upon ice-crystals, which more 

 particularly throw light on the cause why cirrus clouds are 

 occasionally iridescent ; but the rest of the account of this 

 accurate observer, and the admirable drawings which he made 

 of the crystals, will also well repay careful study. Thus, if the 

 cloud consist of crystals of Scoresby's fifth genus, or if crystals 

 of the first and third genera are both present, and if the air is so 

 calm that the crystals can remain in the terminal position into 

 which they would come in falling through still air, then we 

 shall have the phenomenon of both a horizontal and a vertical 

 circle through the sun making across ; whereas, if the crystals 

 are of the first genus only, the vertical circle will present itself 

 without the horizontal. I have myself seen the phenomenon 

 in this latter form. Crystals of either the first or the fifth 

 genus, if occasionally agitated so that they will keep fluttering, 

 would give rise to iridescence if of sufficiently uniform thick- 

 ness. 



Captain Scoresby describes lamellar flakes of snow floating 

 in the air and sparkling in the sunbeams, as always present 

 during severe frosts, when the sky is clear. The beautiful 

 appearance they would have is a familiar one in chemical 

 laboratories, when a glass vessel, in which precipitated tabular 

 crystals are subsiding through the mother-liquid, is placed in 

 the direct light of the sun. The whole liquid then seems 

 alive with minute specks flashing with the brilliant colours of 

 thin plates. 



