14 
rules, as a given proportion of light or sliade produces proportionate 
colours with as much certainty as any figure in arithmetic. 
It may perhaps be convenient in this place to ask, whether it is 
the lamina^ which cause the plates of air of a certain dimension to 
reflect a particular tint or colour, or whether, like the rainbow, par- 
helion, or halo, it may not be produced by the crystallized particles 
of the suijstance acting as water, ice, or hail* ? 
Iris-like tints are to be produced many ways, and sometimes with 
very remarkable pha?nomena. At Weather-Cot Cave, in Yorkshire, 
it is said that if, at certain seasons, a jDcrson placed at the mouth of 
the cave look at the stream or fall of water that is precipitated from 
above it, he will see six or more iris-like appearances. These are 
probably governed by the shadow of the prominences of the rock, or 
perhaps the disturbed undulations of the water: either repetition 
seems to depend upon something analogous to the efi'ect of dark 
lines, which will be seen in the present theory, and the same with 
the repetitions in different substances, and when artifically contrived 
by convex or plane glasses, &c. or perhaps in this latter it may 
depend upon the grinding of the glass, which, however finely po- 
lished and perfectly smooth to our perception, is nevertheless full of 
the irregularities left by the tool of the workman in turning it to 
grind or polish it ; and this would greatly agree with the idea of the 
crystalline particles producing the colours in various minerals; and 
it may seem that a double combination of the watery particles, or 
mist and shadow, produced the circles around the shadows of the 
heads of sir Joseph Banks and his companions, in the bason of the 
boiling fountain of Geyer, when visiting Iceland in l/Tlj as Von 
* As it seems somewhat doubted, what is the form of ice in its crystallization, we 
can assure our readers that we have found snow in regular tetraedrons, Brit. Min. lai. 281, 
and in hexaedtal pyramids, produced by truncation at the edges of the tetraedrons j 
sometimes truncated at the top ; in hexaedral columns formed of plates curiously stel- 
lated with spiculae; and hail in regular tetraedrons, somewhat plated with nuclei: I 
have also found the tetraedron with rounded faces, somewhat like the sulphate of cop- 
per, British Mineralogy, tab. //, but nearer round, like a drop of water. 
