100 Mr B. Martin’s Experiments on Island Crystal . 
culiar structure of the parts of Island crystal, yet unobserved, 
or some particular modification of the particles of light which 
Sir Isaac Newton himself was not apprised of, must certainly 
make a part of their latent cause. How far those numerous 
fine fissures which may be seen in some polished pieces, whose 
planes are all at right angles to the plane of perpendicular refrac- 
tion FCGH, and have the same inclination with the line CF, 
and which are beautifully distinguished by rings of coloured 
light ; I say, how far such fissures may be concerned in pro- 
ducing such a diversity of refracting properties, must be left to 
future inquiry ; only it may be observed, that each fissure se- 
vers from the parallel opiped an equiangular prism , whose base 
is similar to the equilateral triangle FLg’, and its refracting angles 
equal to FLg or Fg-L, viz 39° 4t. (See Fig. 1.) 
“ As these fissures have never been mentioned by Hugenius 
or Newton, and as I have met with but one piece that will shew 
them very plain and distinctly, in order that some idea may be 
formed of them, they are represented in Fig. 8. by the several 
dotted parallel lines , such as their intersections appear to make 
with the surface of the crystal ; and at right angles to the plane 
of refraction DE.” 
(To be continued ,) 
Art. XXIV. — On the Construction of Polyzonal Lenses and 
Mirrors of great magnitude for Light-Houses and for Burn- 
ing Instruments , and on the formation of a great National 
Burning Apparatus. By David Brewster, LL. D. F. R. S. 
Lond., and Sec. R. S. Edin. 
In the year 1811, I had occasion to pay particular attention 
to the construction of lenses and mirrors, of a large size, for 
the purpose of burning, and in the year 1812, I published the 
results of my inquiries in the Article Burning Instruments, 
in the Edinburgh Encyclopaedia, 
The obstacles which are opposed to the construction and 
use of lenses, of a large size, arise, 
1. From the difficulty of casting, grinding, and polishing 
them. 
