688 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of this is to enable one of the tubes to be brought in a line with the 

 optic axis, when the Microscope is to be used as a monocular. The 

 tubes are clamped in any given position (whether for monocular or 

 binocular use) by a screw working in the short vertical piece shovp-n 

 behind them. The screw at the to]3 of the standard acts on the stage 

 and forms the only adjustment for focus. The isophotal prism slides 

 inside the crossbar and is pushed forward over the objective, or with- 

 drawn again when not required to be used, by the screw at the back of 

 the bar. 



There is a second body-tube also of peculiar construction (fig. 77) 

 which has three tubes. The two outer ones are for use with the bino- 

 cular prisms, while the central one serves for monocular observation. 



Blix's Microscopes for measuring the radii of the curved surfaces 

 of the eye.* — Dr. M. Blix uses two compound Microscopes for 

 measuring the radii of the curved surfaces of the eye. Hitherto the 

 surfaces have been consideied as mirrors, in which, the smaller the 

 image of any object, the smaller are the radii. The principle which 

 Dr. Blix makes use of is the following: — 



The image of a point in the axis of a spherical mirror, lies in the 

 axis at a distance from the reflecting surface, which is determined by the 

 distance of the luminous point from the mirror, and by the radius of the 

 latter. If a compound Microscope, which transmits a ray of light from 

 a point in the centre of the plane of the eye-piece to the objective, be 

 placed in the direction of the normal to the reflecting surface and 

 then adjusted so that the point of intersection of the normal with 

 the reflecting surface — the principal point — is focused, then the image 

 of the illuminating point will be at the same place as the point itself. 

 If the centre of curvature of the reflecting surface is then focused the 

 image will again coincide, but this will not take place in any inter- 

 mediate position. If the displacement of the Microscope along its 

 axis in the two cases is measured it will enable the radius of curvature 

 of the mirror to be determined by calculation. 



This result, however, is not so easily obtained in practice, as the 

 field of view is too strongly illuminated by the light reflected from the 

 surfaces of the objective-lenses. Two Microscopes are therefore em- 

 ployed by Dr. Blix, one to transmit the light and the other to observe 

 its image. The axes of these two Microscopes intersect each other, so 

 that the angle between them is bisected by the normal to the reflecting 

 surface ; they can be moved towards the reflector in such a manner that 

 •the jJoint of intersection of their axes coincides with that of the normal 

 and the reflector. The object (e. g. a diaphragm with a punctured cross 

 brightly illuminated) in the field of the first tube, is by the shifting of 

 the tube so adjusted that the image j)rojeGted by the Microscope falls on 

 the reflectiiag surface. By moving the second tube along its axis, the 

 image of the cross is brought into its field also. If the Microscopes 

 are now moved together in the direction of their axes, the image of the 

 cross will disajipear from the field of the second Microscope but will re- 

 appear as soon as the tubes are focused on the plane of the centre of 

 the reflecting surface. By measuring the extent of movement of the 

 tubes, the radius of curvature can be obtained. 



* Zeitschr. f. Instrumenteiik., i. (1881) pp. 381-90 (6 figs.). Cf. also Ceutr.-Ztg. 

 f. Optik u. Meehauik, iii. (1882) pp. 33-4. 



