246 Transactions of the Royal Microscopical Society. 
III. — Further Remarhs on a “ Simple Device ” for the Rlumina- 
tion of Ralsam-mounted Objects for Examination with 
Immersion Objectives ivhose Balsam Angle is 90° or upwards. 
By J. J. Woodward, Surgeon and Brevet Lieut.-Col. U.S. 
Army. 
{Bead before the I^oyal Microscopical Society, October 9, 1878.) 
Since tlie publication of the original paper in the ‘ Monthly 
Microscopical Journal’ for August, 1877 (p. 61), I have been in 
the habit of using a similar prism to that there described, without 
any diaphragm, in the ordinary lamplight illumination of balsam- 
mounted tests for study with objectives of the kind referred to. 
The apparatus is so simple, so easily manipulated, and so effective, 
that a brief description may be of interest. 
A right-angled prism of crown glass (a), the long side of which 
is three-quarters of an inch long by half an inch wide, has its right 
angle truncated as in my “ simple device,” and cemented to a small 
base-piece of brass (b) which is supported in a stiff steel rod 
(c) three or four inches long, and about the thickness of a knitting 
needle. The whole apparatus ought not to cost more than three or 
four shillings. 
To use it, I slip the steel rod into the holder of the rod of the 
dark well usually employed with the Lieberkiihn mirrors. This 
holder fits into the sub-stage of the microscope, and putting a drop 
of oil of cloves on the upper face of the prism, it is racked up into 
optical contact with the lower surface of the slide. I then condense 
the light of a small coal-oil lamp upon the object, through either face 
of the prism, by a small bull’s-eye lens of about three inches focal 
length. One half of this condensing lens may be covered with 
black paper, in which case it can readily be so placed that no ray 
forming a smaller angle with the optical axis than 45° can enter 
the objective. Excellent results can be obtained, however, without 
this precaution, if the intention be merely to resolve a given test, 
without caring to know precisely the angle at which this is done ; 
in this case it is only necessary to place the lamp in such a position 
that a line drawn from its flame to the face of the objective shall be 
perpendicular to either short side of the prism, and condense the 
light in that line. By looking through the other short side of the 
