£54 Mr B, Martin’s Experiments on Island Crystal. 
fraction and a double focus, when ground into lenses ; but they 
differ in other respects, according to their different species. 
“ Before I quit the subject, it may be proper to observe, that 
the original particles of Iceland crystal derive all their won- 
derful powers and properties of acting upon, and variously re- 
fracting the rays of light, from a peculiar disposition and con- 
struction in their concrete , compact , and solid state ; because 
those very particles, when reduced to a fluid state, discover no 
such powers or action on light ; they do not so much as cause a 
double refraction ; they only co-operate with the particles of the 
dissolvent medium in producing a single r fraction of light, but 
the colours are thereby greatly heightened and strengthened; as 
will appear by dissolving \ of an ounce of the crystal in Jths of 
an ounce of aquafortis , and putting the solution (when clear) into 
an hollow glass-prism, and then measuring the refraction, in the 
same manner as for water, and other fluids. 
66 One would think if these strange powers were inherent in 
the particles of the crystal, they would produce these effects in 
every transparent state , whether solid or fluid; and, if you sup- 
pose the cause to be in the particles of light, then why should 
they not suffer the same refractions in their passage through 
them in a fluid, as well as in a solid state ? If the cause be not 
in the matter of the crystal, and of light, it must be owing to 
some special modification of the crystalline particles, in their 
$olid or consistent state, which subjects them to the peculiar 
agency of a spiritus subtillissimus, as above mentioned. 
66 Though all the phenomena of Island crystal may be easily 
experimented with the flame of a candle, or small bright image 
of a window made by reflection from the surface of a convex 
speculum, or from the sun itself viewed through the crystal, 
with a dark glass before the eye; yet the most perfect and ge- 
nuine way, is by placing the prisms and parallelopipeds in the 
rays of the sun in the dark room ; and then the actual separa- 
tion of the rays, and all the variety of colours in each, will most 
beautifully appear to the eye, as they pass across the room, and 
form the numerous images of the sun on the opposite wall, or 
on the ceiling. 
“ The method I take for this purpose is the following: I 
have a box or case about flinches long, fths wide and deep, with 
