Mr B. Martin's Experiments on Island Crystal. 247 
object-lenses of telescopes and microscopes, which I have long 
since constructed therewith. 
“ In a brass tool, whose radius is 4.4 inches, I caused to be 
ground several very clear pieces of this crystal, and they made 
good lenses for Galilean telescopes , as they were hard enough 
to admit of a very true figure and polish. In the same tool a 
piece of plate-glass was also ground. Note, these lenses were 
all of them plano-coiwexes . 
w The solar-focal distances of the glass and crystal lenses were 
determined by measuring them accurately in the sun-beams, 
as follows ; AB (Plate V. Fig. 1.) the glass lens converged the 
parallel rays DA, EC to a focus at F at the distance CF=8.4 
inches. Suppose N the center of the tool, or convexity of the 
lens, and draw NAG, then the angle of the incident ray AD 
will be DAN = ANC, and of the refracted ray AF it will be 
GAF ; and therefore it will be as AF : FN : : sine of inci- 
dence : sine of refraction : : 8.4 : 12.8 : : 3 : 4.56 :: 1 : 1.52. 
“ The plano-convex AB of Island Crystal (Fig. 2.) being 
held in the sun’s ray refracted them to two different focuses , 
viz. F and f The focal distance of the least refracted rays 
was CF — inches. Therefore the sine of incidence : sine of 
refraction :: AF : FN :: 7.J : 11.9 : : 3 : 4.76 : : 1 : 1.586. 
“ The pencil of the most refracted rays terminated atj^ at 
the focal distance C/'=6.1 inches. Therefore the sine of in- 
cidence : sine of refraction : : Af: Nf:: 6.1 : 10.5 : : 3 : 5.16 
:: 1 : 1.72. Sir Isaac Newton makes this ratio as 3 : 5, as we 
formerly observed. 
u Having determined the two focal distances CF and Cf in 
one lens, they are known for any other of a given radius of 
convexity, they being always proportional to the radii of the 
said lenses. For example, in a lens (plano-convex) ground in a 
tool whose* radius is 0.55 of an inch, the focal distances are 0.94 
and 0.76 : because radius 4.4 : radius 0.55 : : 7.5 : 0.94, and 
: : 6.1 : 0.76. 
“ As the lens AB in the present opera has two focuses at the 
distance F f— 1.4 inch, it will require two marks or circles up- 
on the inmost tube or drawer to which it must be drawn out 
and in, to adjust it to e^ch focus respectively. 
