698 General Notes. [ November, 
for lenses of greater angle than this, the illumination would be direct, 
the image being formed exclusively by that portion of the available 
aperture of the lens which was in excess of the equivalent of one hun- 
dred and eighty degrees dry angle. The existence, and even the possi- 
bility, of such lenses was not at that time undisputed, some eminent 
microscopists still maintaining the contrary opinion. But the new re- 
flex illuminator had scarcely arrived in this country before Mr. Samuel 
Wells, of Boston, succeeded in making it act as a direct illuminator, 
with several lenses of at least two makers, an efficient and excellent 
image being formed by these extralimital rays. It immediately occurred 
to Mr. Tolles that a similarly good illumination might be obtained in 
the case of dry lenses, and immersion lenses of moderate angle, by 
changing the angle of the reflecting face so that the illuminating pencil 
should fall just without instead of just within the angle of total reflec- 
tion. He at once made several such prisms, of various angles, but, 
hoping to improve still further upon the plan, refused to offer them for 
sale. One of these was presented by Dr. R. H. Ward at the Nashville 
meeting of the American Association this summer. Its reflecting face 
was inclined twenty-one degrees to the axis, giving an obliquity of forty- 
two degrees to the central reflected ray. This utilizes the extreme 
angle of an objective close up to one hundred and eighty degrees dry, 
or its equivalent in balsam,.and in addition gives a slight illumination, 
though by itself insufficient for useful work, of extralimital rays avail- 
able exclusively to immersion lenses of excessive aperture. With suit- 
able lenses it gives prompt resolution of numbers eighteen, nineteen, and 
twenty of Moller’s test plate in balsam, by lamp-light. The modified 
prisms are inferior to the original for use with lenses capable of taking 
up half the pencil transmitted into balsam by the original “ reflex.” 
The prism of twenty-one degrees gives the maximum efficiency with dry 
mounts, as the extreme capacity of the lens is utilized by the half pencil 
from the illuminator, which can be transmitted to it, —a condition very 
favorable for difficult resolution. Prisms of still smaller angles were 
made, and are now used for lenses of lower angle. The length of prism, 
and the condensing arrangement, are modified to suit the changed angle. 
As in the original reflex, the chromatic aberration is excessive ; and it is 
a question how far the very decided results in resolution attained are due 
to the nearly perfect monochromatic illumination thus secured. The 
new illuminators do not seem to have given results not otherwise attain- 
able, and they are subject to the inconvenience that each one is limited 
to a fixed and comparatively narrow range of angles; but they furnish 
a ready and easy means of oblique illumination, suitable for extremely 
difficult resolution, and entirely independent of thinness of stage and 
concentric rotation of object-carrier. 
Microscorist’s ANNUAL. — The Industrial Publication Company, of 
176 Broadway, New York, having undertaken to publish a list of the 
