ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 299 



Adjustment for diiferent distances between the eyes is eflfccted by 

 the screw D, which moves the eye-piece B', together with the prism h', 

 in a parallel direction. The tubes of the eye-pieces can also be 

 drawn oiit, if greater separation is required. 



The eye-pieces have the usual two lenses, but are of special con- 

 struction in order to equalize the length of the direct axis and the 

 doubly reflected axis, and in spite of this inequality obtain sharply 

 defined images of equal amplification with the same focus. 



Stereoscopic vision is obtained by halving the cones of rays above 

 the eye-pieces. This is effected by stopping off half of the real image 

 of the objective opening formed above the eye-pieces at the so-called 

 " eye-point " (B or jB', which represents the common cross-section of 

 all the pencils emerging from the eye-piece. A cap, with a semi- 

 circular diaphragm, is fitted to the eye-piece (shown in the figure over 

 B'), the straight edge of which, is exactly in the optic axis of the eye- 

 piece, and can be raised or lowered by screwing so as to obtain a 

 uniform bisection of the cones of rays from every point of the field. 



The height of the diaphragm is regulated once for all for the same 

 length of the microscope-tube by finding the position for which the 

 aperture-image (which on withdrawing the eye from the eye-piece is 

 visible as a bright circle above it) shows no parallax against the 

 straight edge of the diaphragm, i.e. so that on moving the eye 

 laterally the image always appears to adhere to the edge. 



In addition to the above caps with diaphragms, the instrument is 

 supplied with ordinary caps with circular apertures, as in B. They 

 taper slightly, and simply slide into the eye-piece, so that they can 

 be readily changed. 



The special feature of the instrument is the ingenious arrange- 

 ment whereby, by simply turning the caps with the diaphragms, 

 orthoscopic or pseudoscopic effect can be produced instantaneously at 

 pleasure. It is more particularly available for tubes of short length 

 for which the Wcnham prism is inapplicable. 



Some discussion subsequently took place as to this instrument,* 

 in the course of which it was pointed out that the device for dividing 

 the rays was similar to that suggested by Mr. Wenham in 1866,f the 

 method of its application, however, and the action of the binocular as 

 a whole being essentially distinct and in fact truly stereoscopic, and 

 not MOH-stereoscopic as supposed, 



A further point raised was % that a crossing of the axes in such an 

 arrangement was essential, in regard to which, however, Prof. Abbe 

 properly points out — what has hitherto not been appreciated — that 

 stereoscopic or pseudoscopic effect does not depend essentially on 

 crossed or not-crossed axes, but upon either the outer or inner halves 

 of the pupils of the observer's eyes being put into action in bin- 

 ocular vision. As the author has dealt fully with this point in the 

 paper read at the January meeting, § we need not refer further to it 

 here. 



* Engl. Mech., xxsii. (1880) p. 323. 



t Quart. Journ. Micr. Sci., xiv. (1866) p. 104, 



i Engl. Mech., xsxii. (1880) p. 352. 



§ See this Journal, ante, p. 203, 



