$48 Mr B. Martin's Experiments on Island Crystal 
“ The images of objects formed in those focuses F andy’will 
be in proportion to their focal distances CF and C f\ and that 
image in the focus F, as it is largest, is also rather the most 
perfect of the two ; and I do not find but that one of these re- 
fractions is as much stated and constant by the lens as the other. 
“ The images being at so great a distance, viz. near 1J inch 
apart, when one is seen distinctly, then only a skim or nebulous 
appearance of the other can be observed ; so that one image 
does not interfere with or spoil the view of the other. 
“ If you draw out the slider to about the middle distance be- 
tween the two marks upon it, then both the images will appear 
imperfectly ; and, in this case, you will observe the images at f 
projected on one side the image F, viz. at <o, in the plane of the 
Field of View; and by holding the drawer very fast and in 
one position, if you turn the case round which holds the lens, 
you will observe the first image at g , to move in a circular 
manner about the image at F, which will appear at rest. And 
it is further to be observed, that, in this motion, the position of 
the image g is constantly the same, or parallel to itself in every 
part of its circuit. 
“ A concave eye-glass of 2, or 2£ inches focus will do best, 
as it gives a large field of view, the principal thing here to be 
regarded, and will magnify as much as a common opera, viz. 
3 or 3 1 times. Such is the construction of a telescope of Is- 
land Crystal best suited, as I judge, for the Amusement of the 
virtuoso in this kind of phenomena. 
“ A microscope constructed in the same manner with a small 
object lens of Island Crystal is seen in Fig. 3. where AB is the 
object lens whose radius of convexity is 0.55 of an inch, and its 
two solar focuses at the distance of 0.94 and 0.76 of an inch, as 
above specified, being a plano-convex . 
“ In my Island Crystal Microscope, the distance between the 
object lens AB and the eye-glass GH, is 6 inches, and the focal 
distance of the glass GH one inch ; therefore, by adjusting the 
microscope to the small object DE, there will be two images 
formed successively at the focus F of the eye-glass, at the dis- 
tance CF = 5 inches. 
“ The lens AB to form the image at F by the least refracted 
.rays, must be at the distance CE — 1.113 inch from the object 
