322 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



They are made of a glycerin mixture ; and it is said tbat the drop 

 of liquid adopts a form near that of a paraboloid or ellipsoid, and 

 thus to a certain extent eliminates spherical aberration ; the curvature, 

 and with it the magnifying power, can be altered by piston- and screw- 

 motions. Such a lens forms a cheap Microscope made on Plateau's 

 principle, with a magnifying power of 100-200. After use the liquid 

 can be withdrawn by means of the piston into a hollow receptacle. 

 The lenses may be combined to the number of two or three to form a 

 system. 



Koristka's Abbe Illuminator.* — Yet another mounting for the 

 Abbe Illuminator has been devised by Signer Koristka, of Milan, for 

 Students' Microscopes, and is shown in figs. 59 and 60. The lenses 



Fig. 59. 



Fig. 60. 



are separated from the diaphragm- 

 holder, and slide in a tube c fixed to the 

 under side of the stage, the upper lens 

 being level with the top of the stage 

 or below it, as may be desired. The 

 diaphragm-holder swings on a pivot, 

 and the diaphragms can be placed 



excentrically by moving the slide P by means of the milled head B. 

 A catch at M shows when the diaphragms are central. The central 

 plate of the holder can also be revolved by the pins underneath it. 



For Microscopes which have a less space than 38 mm. between 

 the centre of the pillar and the stage, and 80 mm. between the base 

 and the stage, a still simpler plan is adopted, the diaphragms being 

 carried in a ring which is movable on the under side of the stage. 



Central v. Oblique Light, f— One of the familiar arts of contro- 

 versialists is to conjure up an imaginary adversary who propounds 

 the most absurd propositions which are immediately demolished by 



* G. Martinotti in Zeitschr. f. Wiss. Mikr., ii. (1885) pp. 500-2 (2 figs.), 

 t Engl. Mecli., xlii. (188G) pp. 451-2 (3 figs.); pp. 527-8 (5 figs.). 



