Mr B. Martin’s Experiments on Island Crystal. 157 
“ And what is still more very remarkable, is, that the same 
piece of Island crystal formed into two prisms, with angles near- 
ly equal, will shew through one angle two images only ; but 
through the other, it will exhibit six. 
“ I have not been able yet to procure Prisms of Island crys- 
tal of more than a sextuple , or sixfold refraction, singly used ; 
but if two prisms are combined or placed together, so as either 
to increase the refracting angle, or diminish it, they become a 
multiplier and a multiplicand , and produce a number of images, 
according to the number in each singly. 
“ Thus, if one prism of two images, and a refracting angle 
of 35°, be 'applied to another prism of two images, whose refract- 
ing angle is SO 0 , the compound prism will then have an angle 
of 65° or 15°, as you please ; but in each there will be four 
images , stronger or fainter, in proportion to the compound re- 
fracting angle. 
“In like manner, if a prism of two images be properly com- 
pounded with a prism of four, there will result a prism of eight 
images , all very distinct, with colours more or less intense, ac- 
cording to the quantity of the compound refracting angle. 
“ Hence also a prism of two images applied to one of six, 
will produce a prism of twelve images. Again, two prisms of 
four images each, make a compound prism of sixteen images. 
And a prism of four applied to a prism of six, produces a 
prism of twenty-four images. Lastly, two prisms of six images 
each, compose one that will exhibit thirty-six images. 
“We have mention made of a double chromatic refraction of a 
prism made of mountain crystal , or crystal of the rock, by Father 
Beccaria, in Phil. Trans, for the year 1762, p. 486. But he 
says it agrees in no respect but number, with the refraction of 
Island crystal ; hence it is plain, this great virtuoso never saw 
a prism of Island crystal ; for if he had, it would soon have 
convinced him of its great superiority, both in regard of its re- 
fractive power and chromatic quality. 
“ But the most extraordinary of all the strange properties of 
Island crystal is next to be rehearsed. It has been a maxim 
generally adopted by all optical writers, that a parallelepi- 
ped of any diaphanous substance, having the refracting power 
equal upon each of its opposite and parallel sides, must there - 
