THE SUN. 69 



of a clear angular polished piece of glass called a prism^ 

 the course of the beam from tliat place will be seen to 

 be bent aside in a direction towards the thicker part of 

 the glass and not only so bent or refracted^ but spread 

 out to a certain degree, so that the beam in its further 

 progress grows continually broader, the light being dis- 

 pei'sed^ into a flat fan-shaped plane : and if this be re- 

 ceived on white paper ; instead of a single white spot 

 which the unbroken beam would ha.ve formed on it, 

 appears a coloured streak ; the colours being of exceed- 

 ing vividness and brilliancy, and following one another 

 in a certain fixed order graduating from a pure crimson 

 red at the end least remote from the original direction 

 (or least deviated), through orange, yellow, green, and 

 blue, to a faint and rather rosy violet. This beautiful 

 phaenomenon the Prismatic Spcctnim, as it is called 

 strikes every one who sees it for the first time in a high 

 degree of purity, with wonder and delight ; as I once 

 had the gratification of witnessing in the case of that 

 eminent artist the late Sir David Wilkie, who, strange to 

 say, had never seen a "Spectrum" till I had the pleasure 

 of showing him one ; and whose exclamations, though a 

 man habitually of few words, I shall not easily forget. I 

 shall not attempt to give any account of the theory of 

 this pj'ismatic dispersicm of the sunbeam ; but an illustra- 

 tion of it may be found in a very familiar and primitive 

 operation the winnowing of wheat. Suppose I had a 

 sieve full of mixed grains and other things shot, for 

 instance; wheat grains; sand; chaft"; feathers; and 

 that I flung them all out across a side wind, and noticed 



