500 
so as to show the steps by means of which I was led to the 
construction of my achromatic prism. 
As a beginner in microscopy, I, like many others, I pre- 
sume, was not blessed with a superabundance of money or, 
at least, not sufficient to keep pace with my enthusiasm. ‘The 
consequence was that I very soon found that I did not see 
all that I desired to see or that evidently could be seen had 
I possessed ampler appliances in the shape of lenses or 
apparatus. Lenses, as magnifiers, were beyond my reach, so 
that I determined to do the best I could with those I had, 
and to assist them to the best of my ability by superadding 
such contrivances for illumination as I could, myself, con- 
struct. As the stand I owned had no means of swinging the 
mirror to one side and thereby making the illuminating 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. 
To do this, I purchased for a few cents and used a simple 
plano-convex lens of short focus and, to fix and centre it in the — 
middle of my rigid stage, I cemented it by its plane side, and 
with Canada Balsam, to the lower surface of an ordinary 
glass microscope slide of the usual dimensions of three inches 
by one. On the upper surface of such a slide the glass bear- 
ing the object rested. A description with ilustrating figures, 
of this simple contrivance I sent to the London Quarterly 
Journal of Microscopical Science in 1857, and it was published 
therein, Vol. v, page 110. The idea, however, of such an illu- 
minator did not entirely originate with me, but had been 
taken from the perusal of a paper by Dr. John Charles Hall, 
“On an easy method of viewing certain of the Diatomacez,” 
in the same periodical Vol. iv, page 205. Dr. Hall employed 
a hemispherical lens used plane side up and with a dark stop 
cutting off the central rays. My contrivance was much cheap- 
er, could be made at home, and permitted the use of unilateral 
illuminating rays. Fora long time this simple illuminator 
answered extremely well, so much so, that by means of it, and 
