$46 Mr B. Martin’s Experiments on Island Crystal. 
with an Object Glass of Island Crystal, and a double Focus. 
$. The construction of a Microscope with a Magnifying Lens 
of Island Crystal, and a Double Focus. 3. The strange and 
singular property of a Prism of Island Crystal, which refracts 
Hays of Light both from and towards the Perpendicular, con- 
trary to all others. 4. An account of a Prism of Island Crys- 
tal, that refracts a beam into twenty different rays of coloured 
light, or exhibits twenty coloured images of one object.” 
“ Preface . — It seems to me that men of the greatest mental 
powers have a limit prescribed to the exertion of them ; else 
how can we account for Hugenius’s and Sir Isaac Newton’s silence 
concerning the optical forms and uses of Island Crystal ? They 
were not only the greatest mathematicians, but the very greatest 
opticians that any age has produced ; and that not only in theory 
but in practice : The former had a large apparatus for grinding 
all kinds of glasses ; and the latter did not think it beneath 
him to employ himself in grinding prisms, lenses and mirrors. 
But not a w T ord in either of these authors about prisms or len- 
ses of Island Crystal; or of its multuple and colorific refrac- 
tion. Sir Isaac mentions the Polishing Island Crystal indeed, 
but in a manner as plainly shews it could be of no use, and it 
does not appear that he ever tried any experiments with a po- 
lished piece in the solar rays, or common light ; for if he had, 
he could not have missed a discovery of most of its amazing 
properties, which make the subject of the present essays ; and 
which would have put him upon a further inquiry into the na- 
ture of light, and the powers of bodies to refract it ; all which 
must now be referred to the age which another Newton shall 
adorn ! 
44 In the first part of this Tract I have given an account of 
several new properties of Island Crystal, and of the various 
modifications and refractions of the rays of light through its 
substance, very different from those which we observe in glass, 
or any other diaphanous subjects hitherto used in optics. To 
these I shall now add some other discoveries which I have since 
made in this wonderful spar, not only in regard to prisms , but 
