160 THE MICROSCOPE. Dec. 
The Achromatic Prism. 
By ARTHUR M. EDWARDS, M. D., 
NEWARK, N. J. 
When a beginner, I was not blessed’ with money to 
keep pace withmy enthusiasm. I soon found that I did not 
see all that I desired to or that could be seen with more 
apparatus. Lenses as magnifiers were beyond my reach, 
so I determined to eek out those I had by contrivances 
for illumination. My stand had no means of swinging 
the mirror to one side and thereby making the illumin- 
ating beam impinge at an oblique angle upon the lower 
surface of the slide. I devised a means of converging 
the almost parallel rays of the sun from all sides upon 
the object and thus getting oblique light, or, in case of 
need, cutting it off from all but one portion of the field 
for the same purpose. TodothisJ purchased fora few 
cents, and used a simple plano-convex lens of short focus, 
and, to fix and center it in the middle of my rigid stage, 
I cemented it by its plane side, with Canada balsam 
(Gum thus is better, being of higher refraction) to the 
lower surface of an ordinary glass microscopic slide of 
the usual dimensions of three inches by one. On the up- 
per surface of such aslide, the glass bearing the object 
rested. Long ago, Dr. John Charles Hall employed a 
hemispherical lens used plane side up and with a dark 
stop cutting off the central rays. My contrivance was 
much cheaper, could be made at home, and permitted the 
use of unilateral illuminating rays. 
It was followed by an illuminator which was made by 
Robert B. Tolles. This is a hemispherical lens of less 
than a quarter inch focus mounted on a brass tube so 
that I can screw it into the part beneath a larger stand, 
Soon after, Dr. Royston Pigott made an illuminator of 
the same construction and found it worked very nicely. 
I added two lenses, one above the other to my first illu- 
