Illumination of Balsam-mounted Objects. By J. J. Woodward. 63 
and the sub-stage is racked up until the drop of oil of cloves is 
spread out into a thin layer, I. 
The object being thus arranged, it is evident that if a beam of 
parallel solar rays (white sunlight), reflected from a plane mirror, 
be thrown through the two apertures upon the face of the prism, 
being perpendicular to that face, it will enter and pass through 
without refraction until it reaches the upper surface of the thin 
glass cover Gr. The parallel rays impinge upon this surface, as is 
evident from the construction, at an aDgle of 45° with the optical 
axis 0 0. If, now, the medium next above the thin cover, Gr, be 
air, this obliquity will be greater than the critical angle, and total 
reflexion of the rays will take place. If, however, the medium 
next above the thin cover be water, the obliquity will not be greater 
than the critical angle. Refraction having taken place, the rays 
will enter the water, H ; and if an immersion lens of sufficient angle 
of aperture be focussed upon the objects mounted beneath the cover 
Gr, these rays not merely enter the front of the objective, but will 
form a well-defined image of the object on a brightly illuminated 
field, which will be visible through the eye-piece of the instrument 
in the usual way. Of course it is evident from the diagram that 
with no dry objective, or any immersion objective of less than 90° 
balsam angle, can anything whatever of balsam-mounted objects * 
thus illuminated be seen. 
Immersion objectives may be divided according to their behaviour 
with this apparatus, into three classes : — 1st. Those with which, since 
they do not have sufficient angle of aperture to admit the illuminating 
pencil, nothing can be seen, precisely as in the case of dry objectives. 
2nd. Those which have sufficient angle of aperture to admit rays 
of this obliquity, but are incapable of bringing them to an image- 
forming focus ; with these the field appears well illuminated, but 
the objects are not well defined. 3rd. Those which not only admit 
rays of this obliquity, but form well-defined images with them. 
To this class belong not merely immersion objectives with the so- 
called duplex fronts, but others ; and I may add, not merely objec- 
tives of American make, but some constructed by a well-known 
English house. As might be expected, the quality of the image 
formed by the direct rays of the sun thrown through a pin-hole at 
this excessive obliquity varies very greatly in different cases. I will 
state, however, that I have thus far found at least seven objectives, 
some of English, others of American make, which define sufficiently 
well under these circumstances to resolve Amphipleura pellucida 
mounted in Canada balsam. With the objectives which performed 
best, the field was of exceeding whiteness and brilliancy, but by no 
* The apparatus can be used, of course, to secure black-ground illumiuation of 
suitable dry objects if they are mounted on the slide instead of the cover, as is 
usual. 
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