Polarized lAglit through Bouble-refrading Crystals. 143 



the dark segments, while the sections of the black rings, especially 

 near the centre of the system, look more Hke straight hnes than 

 circular arcs, and form a system of octagons. 



The effect upon the rings, produced by placing on the stage a 

 film of selenite in the position in which it should give the black 

 bands previously described, is a strange one. Instead of a black 

 band occurring, the coloured ring belonging to that part of the 

 spectrum is seen to split into two. It sends off a branch as it were 

 from its lower part, which shoots across the adjoining black ring, 

 and joins itself with the lower part of the next coloured ring. 

 This last ring then in turn sends ofi" a branch from its middle 

 part, which in hke manner unites with a third ring, which in turn 

 does the same to its neighbour. All this takes place within the 

 space which should be occupied by the black band. 



The beauty of these last experiments, wonderful as it is, may 

 be still further enhanced by the use of a double-image prism as the 

 analyzer. The result is analogous to that obtained with the same 

 arrangement in the case of the selenite previously described. We 

 now get not only two spectra, but two systems of rings, which, by 

 superposing the spectra, may be made to interlace with one another. 

 Wherever a black ring of the one spectrum crosses a black ring of 

 the other, the intersection is of course still black. Where a coloured 

 ring of the one system crosses a black ring of the other, it retains 

 its original colour ; but if a coloured ring crosses a coloured ring, 

 the intersection is of the resultant colour of the two combined. 



Still more complex figures are got by employing two or more 

 crystals in combination. 



Indeed, there is no end to the variety of exquisite beauty, both 

 in colour and in pattern, which a httle ingenuity may produce. 

 Pigments would be almost as helpless as words in representing 

 many of these. The appearance produced by a single crystal with 

 a double-image prism as analyzer, may be not inaptly compared to 

 a tesselated pavement of every colour made for a fairy palace, while 

 that produced by combining two crystals may be said to resemble a 

 suit of chain armour wrought for a fairy king, in jewels of which no 

 two are of the same hue. — Bead before the Boyal Society of Edin- 

 burgh. 



