Mr B. Martin’s Experiments on Island Crystal 159 
Images of the whole of the window-shut, formed upon the 
screen by twelve separate rays of light, regularly refracted 
through the crystal terminated by parallel sides. 
46 But that this most amazing of all optical phenomena may 
be more clearly apprehended, I have represented it to the eye 
in Fig. 9, where the images at 1, 2, S, are those in the line just 
mentioned, and which, in the present piece of crystal, becomes 
doubled in the images 4, 5, and 6. This line is refracted also 
sideway , so as to form three other images on each side, as at 7, 
8, and 9, above ; and at 10, 11, and IS, below ; and all in so 
regular a manner, as to form the mathematical figure or rhom- 
bus, every way similar to that of the plane of perpendicular re- 
fraction CFHGr. 
44 These images are all of them tinged with variety of colours, 
except the two central ones at S, and 5, which appear nearly as 
white as before. They are not equally coloured, however, for 
some are almost wholly red, others yellow, others green, blue, 
or violet, according as you vary the inclination of the surface to 
the incident ray. The two images at 9, and 10, in the acute 
acute angles are very faint, and, unless the object be very 
bright, cannot be seen, as the sun, a candle, &c. 
44 If you turn the crystal round an axis, the whole system of 
images moves round with it, as in other cases ; constantly with 
the red part of each image towards the central hole or beam, 
and the violet the most remote from it. 
44 If prisms of two, four, or six images be successively applied 
to the parallelopiped of crystal, they will multiply the number 
of images accordingly in the rhombus, and produce the number 
twenty-four, forty-eight, and seventyAwo images of the sun or a 
candle ; the greatest part of which will be very distinct, and 
completely tinged with regular prismatic colours, so as to compose 
a kind of natural girandole of painted luminaries, infinitely ex- 
ceeding any production or imitation by the art of glass. 
44 Besides these now recounted, there are other, and perhaps 
more strange refractions of Island crystal ; but to give a detail 
of all the properties that may be observed in this substance, and 
the multifarious manner in which it acts upon light, would be 
very tedious ; especially as I cannot account for any of those 
already related upon the common principles of optics . Some pe- 
