Balsam-mounted Objects. By J. J. Woodward. 247 
prism the observer can readily see when the face of the objective is 
best illuminated. 
For the illumination of immersion objectives which do not 
much exceed 82^ balsam angle, I sometimes also use in the same 
way a prism of 98^ angle, and get thus very good results. I have 
sent one of each of these prisms to Mr. Mayall, with the request 
that he will exhibit them to those interested. 
Prisms of various other angles may be used, and illumination 
of any desired obliquity thus secured. It may be laid down as a 
principle that such delicate striae as those of Amvhipleura 
jpellucida will be most strikingly separated by any given objective, 
when the most oblique pencil it can admit without distortion of the 
image is used. But, practically, I have found as yet no objectives 
which bear, without distortion of the image, a much greater angle 
of illumination than can be obtained with the 90° prism, and for 
objectives of lower balsam angle than 90°, but greater than 82°, 
the 98° prism answers quite as well as one of more nicely adjusted 
intermediate angle. Immersion objectives of still lower balsam 
angle than 82°, which cannot be illuminated even with this prism, 
may be rejected as unworthy serious consideration at the present 
day. 
The two prisms just described will be found, then, to answer 
all practical uses with our present objectives, and I think it will 
probably be cheaper to have two separate prisms than to grind a 
single one, the two short sides of which should be respectively at 
angles of 45° and 49° with the optical axis, which would of course 
answer the same purpose. 
In this connection, I may remark that the general recognition, 
in the United States, of the great angles obtainable for immersion 
objectives, several years ago suggested the importance of immersion 
condensers to our instrument-makers. Mr. E. B. Tolies wrote in 
1871 to the ‘ Monthly Microscopical Journal,’ * at the close of an 
article on the aperture question, “ Certainly the use of immersion 
condensers is abundantly indicated in the above simple experi- 
ments,” and, acting on this indication, he has manufactured a 
variety of plano-convex, nearly hemispherical, lenses, semi- 
cylinders, &c., to be placed, with immersion contact, beneath the 
slide for this purpose. 
Besides these devices and various experiments with immersion 
objectives placed beneath the stage in lieu of the ordinary achro- 
matic condenser, considerable popularity has been enjoyed in the 
United States by Wenham’s reflex illuminator, which its dis- 
tinguished inventor designed to give black-ground illumination for 
objects mounted dry,t but which Mr. Samuel Wells, of Boston,^ 
* Vol. vi. p. 36. 
t Same Journal, vol. vii., 1872, p. 236. 
