*jZ*rua\ ju^fS'] Royal Microscopical Society. 279 



indistinct. The focussing motion is given by tlie two large milllieads 

 moving the stage to or from the body of the instrument by a rack and 

 pinion. The convenient position for observation is secured (as hereto- 

 fore) by tilting the whole instrument on the trunnions in the uprights, 

 through which (in this case) rims the pinion actuated by the two large 

 millheads for moving the stage. 



" The Illuminating Lens. — Immediately under the stage is a lens of 

 large angular aj)erture, achromatic or not, receiving its illumination 

 from the usual mirror situated underneath, and by a stop of suitable 

 form the light from the lens is directed through any transparent 

 object. 



" Tlie Prismatic Illuminator.-^-This is shown at Figs. 7 and 8. It is 

 made from a rectangular or isosceles prism of glass, by working a con- 

 vex or plane face for the incident surface, and cutting the reflecting 

 surface into two planes at the binocular angle, the emergent sui'face 

 being left plane. Its convex siu'face, when placed at its focal distance 

 from a flame, renders its light parallel, which the two inclined surfaces 

 reflect up to the two halves of the separated object-glass when the prism 

 is mounted in an adjustable frame under the stage. If the incident 

 surface of the prism is plane, the com-se of the light is unaltered in its 

 first direction, and therefore not condensed, as in the case of the prism 

 having a convex incident surface. By the use of this prism the xxsual 

 mii-ror is dispensed with altogether, and a far more brilliant and dis- 

 tinct light secured. 



"Stops. — To prevent false light it is necessary to stop-off the central 

 pencil from the illuminator by a stop of black paper or metal placed 

 between the halves of the object-glass. This stop may or may not 

 have a prolongation, so as to enclose and prevent the entry of light 

 into the angular space between the separated body-tubes, where long 

 draw-tubes are not used for that end. It is unnecessary to use a 

 divided object-glass for a monocular vision, as any of the ordinary 

 powers supplied answer for that purj)ose, but should the closed divided 

 glass be so required, a narrow stop must be placed so as to prevent the 

 direct light from coming thi'ough the slit. 



" A reference to Fig. 4 will serve to explain the optical arrangement 

 of this microscope, where each half of the object-glass occupies an 

 equally angular position with respect to the object, and is thus capable 

 of making a distinct image of the object of the same intensity for each 

 eye visible thi'ough the two eye-pieces, as shown, the mirror supplying 

 through the illuminating lens the light for transparent objects, or the 

 same may be lighted by the prismatic illuminator without a mirror, 

 while for opaque objects the light is received from a reflector above, 

 or by preference from one side. 



" Figs. 5 and 6 explain themselves, as showing a method of applying 

 this divided system to microscopes of the usual make wherein the focus- 

 sing is done by moving the body of the instrument instead of the stage 

 as previously described. 



" The eye-pieces most suitable for use with the divided object-glass 

 are of the inverting or erecting kind, according as to whether the instru- 

 ment is used as a simple binocular, or as a stereoscopic microscope." 



