“journal, Aug. 1, 1870. Royal Mieroseopical Society. 63 
tention to two other advantages which are, in my opinion, of con- 
siderable importance. 
The first of these is the much smaller angle at which the two 
tubes, constituting the body, converge, which arises from the fact 
that the object being erect, it is no longer necessary that the pencils 
of light should intersect each other, for the purpose of avoiding 
pseudoscopic effects, as in the Wenham form ; the consequence being 
that in the erecting binocular this angle is reduced from 121 to 
8 degrees, and the points towards which the eyes converge are 
situated at the respective distances of 10} inches and 162 inches, 
which distances are proportional to the cotangents of the two 
angles. 
Most of us have probably found that persons unaccustomed to 
the use of the binocular complain of a straining and aching of the 
eyes after using the instrument for a few minutes only, and this 
would probably be found to be the case principally with long- 
sighted persons, with whom the convergence of the eyes is gene- 
rally extremely small, and to whom consequently the abnormal con- 
dition of vision at a short distance is somewhat painful. 
The only remaining peculiarity in the instrument is the rotation 
of the body on the axis in which the prisms are placed, by which 
means the stand can, by half a revolution, be placed completely out 
of the way of the observer, or by its rotation to a greater or less 
extent the student can place himself relatively to any object on the 
stage in any position which he may deem the most expedient for his 
purpose ; or, again, when two or three observers wish to examine 
specimens of interest they can do so successively without rising 
from their seats. 
In conclusion I would only remark that although I have given 
the angles of the upper and lower prisms as being 90 and 75, and 
have stated that the hypothenuse planes are placed together, it must 
by no means be assumed that these angles and positions are the 
best possible; it may be that as good or even better results might 
be obtained were the upper prism placed beneath the lower, or were 
the positions of the lower prisms reversed (causing a still smaller 
convergence of the tubes), or their angles different from those which 
I have described, but it appears to me that the excellent definition 
of the present arrangement leaves little to desire in an erecting 
binocular microscope. 
F 2 
