ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 557 



directly transmitted by the plane prism-face may be thought to 

 produce the purest effect of oblique illumination ; but in our ex- 

 perience wherever oblique light is required for the resolution of 

 stria?, &c, the slight condensation of rays produced by the curved 

 surface of the hemispherical lens is no detriment, but rather the 

 contrary, whilst for facility of manipulation the lens is greatly to be 

 preferred. 



Mounting the hemispherical lens on a plate to be put immediately 

 beneath the slide is not to be commended, for every movement of the 

 slide then carries the illuminator with it, and the direction of the 

 light requires continual readjustment. No better plan of applying 

 the lens has been suggested than that adopted by Tolles and Ross, in 

 which it is mounted to slide or screw into the stage-aperture from 

 beneath. This plan is applicable to nearly all the modern Micro- 

 scopes having mechanical stages. 



For the Microscopes generally used on the Continent, without 

 mechanical stage-movements, the hemispherical lens may be mounted, 

 as suggested by Professor Abbe, in a disk of metal made to drop into 

 the stage-opening from above so that the plane face is flush with the 

 level of the stage. 



Radial Tail-pieces. — Since the introduction of the Zentmayer 

 swinging tail-piece or swinging substage in 1876, several opticians 

 have carried out the same principle, but instead of the pivot motion of 

 Zentmayer a disk is applied at right angles behind the stage in which 

 a movable zone is fitted to carry the tail-piece. In all the Micro- 

 scopes we have inspected in which this plan is adopted, we remark 

 that the attachment of the tail-piece is so slight as seriously to inter- 

 fere with the firmness of the substage. This is a great inconveni- 

 ence in all manipulations of the substage ; the rackwork, centering- 

 screws, diaphragms, mirror, or whatever may be attached to the tail- 

 piece, cannot be touched whilst the eye is directed through the 

 Microscope, without the flexure of the tail-piece causing the illumina- 

 tion to move from the field of view. Of course this applies only to 

 the use of high powers, but all such Microscopes are supposed to be 

 made specially for high-power work. 



Electric Light in Microscopy. — Referring to his previous paper 

 on this subject,* Dr. Van Heurck sends us the accompanying Fig. 103, 

 showing the Regnier battery which he has adopted in place of the 

 Tommasi ; the sulphate of copper being placed in the small basket 

 at the left-hand side of the cells. 



The Regnier accumulator is also shown in Fig. 104. 



Dr. Van Heurck adds that the Regnier battery can be placed in 

 the laboratory of the microscopist, as it does not give off any vapours. 

 It will remain charged for at least a month if sulphate of copper is 

 added as required. Sixty-four Regnier elements (each = 1 ■ 07 volts), 

 charging sixteen accumulators, lighted a great part of his house for 

 six weeks. They can be used with only one accumulator, to act as a 



* See this Journal, ante, pp. 41S-20. 



