ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 75 



atus intended for observation with both eyes. It consists of two Micro- 

 scopes combined on a single stand. Each of these Microscopes possesses 

 a complete optical equipment : mirror, illuminating apparatus, objective, 

 and ocular. The stage is unusually large so as to receive two preparations. 

 The coarse adjustment, which is by rack-and-pinion, acts simultaneously 

 on the two combined tubes. Two fine screws between the tube and 

 objectives serve as fine-adjustment. For regulation of the ocular width 

 an arrangement is adopted similar to that used in the Greenough 

 Microscope, and, as in the same instrument, the images are directed up- 

 wards by Porro prisms. The upper tube parts, containing the Porro 

 prisms and the oculars, are both movable. The result is that the optical 

 axes are moved • parallel to one another and thus impinge on the ob- 

 server's ocular width ; both images therefore enter his eyes. The 

 images overlap in accordance with that wonderful property of the eye, 

 whereby an image received from the one eye is conveyed by means of a 

 central nerve station to the other. But these images are usually not 

 alike. If the objects can be so arranged that the resultant images group 

 themselves separately in one field of view, both objects can be compared 

 without any further precautions. If the objects do not lend themselves 

 to this, as indeed is usually the case, the semicircular stops are applied 

 to both oculars to stop out half of each field of view in such a manner 

 that in the eye two semicircular fields form a complete circular field, in 

 which both objects separated by a scarcely visible line of demarcation 

 can be observed and compared. If it be desired to observe consecutively 

 both complete images in quick succession, the stops can be opened and 

 shut by a left and right movement. This property makes the double 

 Microscope especially adapted for the comparison of healthy and un- 

 healthy organs, or of adulterated and normal foodstuffs. It would be 

 also possible to compare two objects under different conditions of 

 magnification, of illumination, of bright and dark ground, of ordinary 

 and polarized light. With the usual accessories the instrument could 

 be used as a penological Microscope for the examination of minerals. 

 Its application to colorimetric and spectroscopic tests readily suggests 

 itself. It is, moreover, pre-eminently suitable for stereoscopic observa- 

 tions. 



Fig. 8 shows this instrument externally. 



(3) Illuminating- and other Apparatus. 



Zeiss Pocket Refractometer for Mineralogists and Jewellers." 

 F. Lowe describes this instrument, which has been constructed at 

 the suggestion of the gem expert W. Ran, and which involves an 

 adaptation of the Bertrand-Leiss refractometer. The principle of this 

 refractometer depends on observing the angle of total reflection of rays 

 incident on a face of the crystal, the rays having previously passed through 

 a flint-glai-s hemisphere horizontally placed and rotatory about its vertical 

 axis. Fig. 9 shows a portable form of the instrument. The mirror $p 

 is adjustable and reversible, so that it can be used for downwards or 



* Zeitschr. f. Instrumeutenk., xxxiii. (191:!) pp. 108-11 (2 figs.). 



