156 Mr B. Martin's Experiments on Island Crystal . 
By these prisms it appears, that there is not only a double 
but a multuple refraction in Island crystal ; for some of these 
prisms held in a beam of the sun's light, will separate into two on- 
ly, each of which produces a coloured spectrum of the sun much 
larger, and the colours more vivid than in those of glass prisms of 
the same refracting angle. And the refraction is so nearly equal in 
the two beams, that scarce any difference can be seen in the co- 
loured images they make of the sun, a candle, or other object. 
“ Again, some prisms separate the solar ray into four parts , 
or particular beams of coloured light, which make four spec- 
trum's of the sun, all nearly equally coloured and strong. These 
prisms make every object seen through them appear four , and 
each one coloured, as in a glass prism. 
“ Some other pieces of Island crystal afford prisms with a 
sextuple power of refraction % for they will divide a beam of the 
sun’s light into six separate coloured beams , and consequently 
shew as many coloured images of the sun, and of luminous bo- 
dies viewed through them. But these pieces of crystal are not 
very common ; I have seen but one that was clear enough to 
make a good prism. 
M The extraordinary refractions of these different prisms are 
represented in Fig. 5, 6, 7. 
“ In Fig. 5, through a small hole H in a window-shut ABCD, 
a beam of the sun’s light HE is transmitted to the prism of 
Island crystal ahc , and is thereby separated into two parts EF, 
EG ; and these again refracted into the air, make the two di- 
verging rays of light GI, FK, in all the different degrees of 
refrangibility ; and these paint the two images of the sun I, K, 
in colours very intense and lively. 
“ The same beam of light HE in Fig. 6. is divided into four , 
and these produce four very beautiful spectra , and all of an 
equal degree of light and tints, nearly as at I, K, L, and M. 
“ In Fig. 7, the same beam is divided into six others, which 
depict six coloured images of the sun upon a screen ; and notwith- 
standing the same quantity of light is now divided into six co- 
loured spectra , which, in a glass prism of the same refracting 
angle, would make but one, yet it is surprising to observe how 
strong each spectrum appears in regard both to brightness and 
Colour, and how little it falls short of that in glass. 
