Mr B. Martin’s Experiments- on Island Crystal . 249 
ED ; but at that distance the image by the most refracted rays 
will be formed at f at the distance Cf— 2.44 inches, hence the 
distance between the two images at f and F will be Ff = 2.56 
inches. 
“ Therefore, in order to view the image made by the great- 
est refraction, the lens AB must be placed lower at a b, to make 
the image ^rise to F, the focus of the eye-glass GH. The dis- 
tance A a is about J of an inch. 
“ The button or cell, holding the lens AB, has a horizontal 
motion, and, by turning it round, the two images will be seen to 
circulate round a common center, and also one about the other, 
yet so that they always keep parallel each one to itself, and both 
to one another. 
“ I shall only add, that I find, by experience, the best method 
of making those experiments is by making one fine hole (or 
more) with a pin in a black spot of a card, (a club or spade ,) 
and laying it under the lens, you reflect a strong light through 
it ; which illumined hole makes two excellent images for this pur- 
pose. 
“ After the same manner you may make five small holes in a 
right line, in the compass of T A n th of an inch ; these five holes well 
illuminated will afford ten images, which will appear in two rows 
near together ; then, upon turning the cell round, you will ob- 
serve all the ten images in motion in a very curious manner, 
one rank moving up, then turning round the other, after this 
they traverse between, and go down ; and thus keep mutually 
and regularly moving among each other, they in some measure 
make a microscope dance of images to the no small divertisement 
of the Tyro in Optics. 
“ Thus much for the construction of a telescope and micro- 
scope with Island crystal. We now return once more to spe- 
culate the wonderful properties of this substance formed into 
prisms* that have not yet been mentioned, or ever before ob- 
served. It has been hitherto allowed that a ray of light passing 
out of a dense solid medium into one that is rarer, will always 
be refracted from the perpendicular in that point ; and this law 
of Nature was never found to have any exception in all com- 
mon refracting crystals, Glass, &c. nor even in Island crystal, 
till since I published my first essay upon it. 
