Afonthly MJcroscoplcan 
Journal, Feb. 1,1869. J 
for the Microscojpe, 
113 
are not given as radii, but as the diameters of the circles in 
thousandths of an inch — for I thus note them down for the con- 
venience of making and finding the steel gauges and to prevent 
division into two-thousandths, which would frequently occur in the 
corrections. The following are the curves : — Back triple, — Posterior 
of crown, •812 ; three next surfaces, crown and concave flint, '440 ; 
front flat, diameter of lens, '173 ; density of flint, 3*630; ditto of 
crown, 2-437. 
Curves or templates of middle : — Back, • 233 ; contact surfaces, 
• 233 ; front, 1 J inch, or fth inch radius ; diameter of lens, * 138 ; 
density of flint, 3 • 686 ; ditto of crown, 2-437. 
Single front of crown, "100 or -^ih. template ; diameter of lens, 
•930 ; thickness, '570, measured from the top ; density, 2-437. 
The focus, or magnifying power, of the two back combinations 
is very nearly equal, and each 4f times that of the single front ; 
for I have found that if the middle is of shorter focus than the 
back, that it is difficult to obtain satisfactory correction. The lenses 
are fitted into their cells without shoulders, as their diameter is 
only just sufficient to admit the full pencil of rays, and their sur- 
faces are utilized to the extreme edge, a desideratum that can 
always be secured by a proper mode of working 
{To be continued.') 
