ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
383 
shown in figs. 34 and 35. B is the supporting column, and the 
principal axis, which contains a second axis carrying the second arm 
A^^, to which the two Microscopes Mj and are screwed. The prin- 
cipal arm A^, connected with the axis a and carrying the telescope, has 
two prolongations, a and /?, which extend beyond the divided circle, ft 
is connected with the fine-adjustment screw F on the clamp k ; and a 
carries the micrometer screw S. S serves to turn the Microscope arm 
A^^ alone ; while F moves the principal arm with the upper part of the 
instrument, and at the same time the Microscope arm. A second arm is 
not necessary on the vertical circle, where it is replaced by the telescope 
carrier. The circles being divided into one-third of a degree, a com- 
plete turn of the drum is made to correspond to 20 minutes, and it 
is divided into 200 parts, so as to read to tenths of a minute. 
(2) Eye-pieces and Objectives. 
Binocular Eye-pieces. — The late Mr. R. B. Tolles’s binocular eye- 
piece has not yet been described in this country, and as inquiries are 
constantly made on the subject, the following description is reproduced 
with slight modifications from his original paper.* 
“ To apply the binocular principle to the eye-piece of a Microscope or 
telescope, it is only necessary to make use of the erecting form of eye- 
piece, and to place the dividing prism at the point where the pencils 
composing the whole bundle of rays proceeding from the object cross 
the eye-piece, which is the point where, in any erecting eye-piece, the 
diaphragm proper is correctly placed. 
If the theory of the erecting eye-piece of common form were generally 
understood, no demonstration that binocularity can be given to such an 
eye-piece would be necessary. Suffice it to say that, since any one pencil 
of light proceeding from any point of the object through the whole area 
of the object-glass does at this point equally fill the whole area of the 
diaphragm (that being of proper aperture) substantially in the same 
manner, therefore the division for binocular vision, if made here by the 
appropriate prism, must be a very nearly equal division of every par- 
ticular pencil, and give a similar and satisfactory image of the entire 
field in each eye-tube. This is a sufficient expression of the whole 
theory of the binocular eye-piece. 
It is, however, important in order to avoid pseudoscopic effects, to 
adopt the proper form of dividing prism ; and this form is precisely 
that best suited to that kind of binocular Microscope in which the 
dividing prism is placed immediately above the objective. The natural 
presumption has been — contrary to this — that prisms of rectangular 
form would give the proper effects in the eye-piece, because of the 
pseudoscopic effects produced by their use in the Microscope of binocular 
body. But this is an error, inasmuch as the pencils proceeding to form 
the second image in the erecting eye-piece reach the small dividing 
prism under conditions suitable for correct vision of the object loere the 
eye placed there, and, accordingly, the same false appearances obtain 
with the eye-piece of rectangular prisms, having oculars above, as if such 
division were made immediately above the objective ; the effect being, 
* Sillimau’s American Journal of Science, xxxix. (1865) [>p. 212-5 (1 fig.). 
