Mr B. Martin’s Experiments on Island Crystal. 155 
If the two pieces are so posited, that the planes of perpendi- 
cular refraction are at right angles with each other, then the ray 
KL, which before was refracted regularly, will, in this case, suffer 
an irregular refraction : and the ray IN before irregular, will 
now be regularly refracted through the lowest crystal, but still 
they pass singly through it. 
“ If one of these two pieces has its position inverted, or hf 
placed upwards, then will the ray IE be refracted to L, parallel 
to BD ; and DK will be refracted to the same point L, in a di- 
rection parallel to BE in the first piece ; and, consequently, by 
two pieces of crystal in such positions, no image of any object 
can appear double. 
“ In all other positions of the refracting planes besides those 
now mentioned, the rays DK, El, will be separated each of 
them into two , in passing through the lower crystal ; and, there- 
fore, any object seen through two crystals so posited will appear 
quadruple , or will have four images. 
u These are the principal properties of Island crystal enume- 
rated by Hugenius, and from him by Sir Isaac Newton. But 
neither of them say a word of any colorific refraction in Island 
crystal, nor mention any thing more than a double refraction , 
and that through parallel surfaces only *. But this substance 
has still many, and much more wonderful properties, than those 
above mentioned, and are commonly known ; for Island crystal 
has a much stronger colorific refi'action than glass ; and it is no 
wonder, since its refracting power is so much greater, being, as 
we have said, in the ratio of five to three , whereas that of glass 
is but as three to two. 
“ Though Island crystal is of a tally nature, and much softer 
than glass, yet it will take a polish little inferior to that of glass ; 
at least it answers for all purposes and experiments of prisms, 
several of which I have made, and shewn in my public lectures 
for many years past. 
* Bartholinusobserved the sextuple refraction (See his Exp. Cry stall. Island. 
p. 39. Exp. xi.) ; and Huygens noticed the colours in the supposed fissures, paral- 
lel to the long diagonal. See his Traitc de La Lumierc , p. 95 — Ed. 
