THE 
MONTHLY MICROSCOPICAL JOURNAL. 
AUGUST 1, 1877. 
I . — A Simple Device for the Illumination of Balsam-mounted 
Objects for Examination with certain Immersion Objectives 
ivhose “Balsam Angle ” is 90° or upwards. By Surgeon 
J. J. Woodward, Brevet Lieut.-Col. U.S. Army. 
( Taken as read before the Royal Microscopical Society, June 6, 1877 .) 
Certain immersion objectives are so constructed that they are 
capable of admitting rays which enter the front lens at a greater 
angle with the optical axis than the limit for dry objectives. That 
this is not only theoretically possible, but that such objectives have 
been successfully constructed, was several years since demonstrated, 
in the ‘ Monthly Microscopical J ournal/ both by Mr. Keith and 
myself,* notwithstanding which, the contrary has often since been 
energetically asserted by writers in the same Journal. 
Meanwhile, immersion lenses possessed* of the excessive angle 
in dispute, continue to be put into the market by more than one 
maker ; and perhaps some of the purchasers will be interested in a 
simple device which I have used for some time with such objectives 
to illuminate test-objects mounted in balsam. This device consists 
merely of a right-angled prism of crown glass mounted beneath 
the stage in such a manner that its long side can be connected by 
oil of cloves, or some similar fluid, with the slide on which the 
object is mounted. The details of the plan will be understood from 
the diagram on next page, in which the glass prism is seen in 
section just beneath the object-slide F F. J ust below it is another 
right-angled prism, of the same dimensions, made of brass; the 
section of this prism is indicated by dark shading in the diagram. 
The right angles of both prisms are truncated, and the facets are 
cemented together in such a manner that the long sides of the 
prisms are parallel. The brass prism slips transversely in a groove 
in the top of a holder, C, which is fitted into the sub-stage of the 
microscope. D D is a blackened brass screen held in position by 
two brass arms, one of which is shown in the figure. This screen 
is parallel to the adjacent face of the glass prism, and has in it a 
small circular aperture, E, about the size of a large pin-hole. The 
side of the glass prism next the screen is covered with black paper 
* June 1S73, p. 268; November 1873, p. 210; March 1874, p. 119; Sep- 
tember 1874, p. 124. 
VOL. XVIII. 
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