TKANSACXIONS OF SECTION A. 563 



ascertain the time-rate of this decrease, in order if possible to find what th« true 

 potential-diflference was when the surfaces were perfectly clean, as compared with 

 a DanieU cell. A represents one series obtained from commercial sheet zinc and 

 copper by Thomson's method, and B another from redistilled zinc and commercial 

 copper by Kohlrausche's method : — 



A. Minutes after cleaning 9 51 74 108 

 Potential-difference -SOD '710 -670 -660 



B. Minutes after cleaning 13 21 74 115 185 3,700 

 Potential-difference -820 -SID -770 -750 -740 -640 



From these it is evident the potential-difference would not be far under -90 at 

 the time of cleaning, and considering that, even when just cleaned, the plates cannot 

 be considered quite clean, this value corresponds very well to the ratio of the differ- 

 ence of the heats of combination of copper and zinc with oxygen to the heat-equi- 

 valent of a Daniell cell — according to Thomsen -90 . 



AVhen the quadrants were wet with distUled water, the potential-difference was 

 •82D to '880. When the copper was wet with copper sulphate solution, and the 

 zinc with zinc sulphate, thus constituting the analogue of a Daniell cell, the differ- 

 ence of potential was '98 Daniell. 



The experiments seem to support the view, that the potential-difference of sub- 

 stances in contact is dependent on the chemical action going on at their exposed 

 surfaces. 



071 a Wave Apparatiis for Lecture purposes, to illustrate FresneVs 

 conception of Polarised Light. By C. J. Woodward, B.Sc. 



10. On a Microscope vjith arrangements for illuminating the suh-stage} 

 By Edward Crosslet, F.B.A.S. 



The light from the lamp is thrown into the hollow horizontal axis of the 

 microscope with the aid of the bidl's-eye condenser, and by a prism placed in the 

 centre of this axis, is reflected forwards in the direction of the axis on which the 

 swinging sub-stage turns. 



The arm of the swinging sub-stage is made in the form of a box, and carries a 

 second prism in the axis on which it moves, so as to intercept the rays of light 

 coming from the first prism, and reflect them in the direction of the arm or box. 



At the end of the box is a third prism, which throws the rays of light forward 

 on to the mirror by means of which they are finally directed to the object on 

 the stage. 



No change in position of the microscope on its horizontal axis affects the 

 direction of the light from the lamp ; and whatever the position of the swinging 

 sub-stage, whether above or below the stage, the illumination remains constant 

 iipon the object. Thus the greatest facility is given for illuminating the object at 

 any angle, and seeing which is most suitable. 



The prisms are 1-inch, and give sufficient light for a ^in. object-glass with a 

 Ross B eyepiece, using, of course, a suitable condenser beneath the stage. The 

 field of a 4-inch object-glass is also fully covered. 



I 



11. On a Nevj Polarising Prism.^ 



By Professor Silvanus P. Thompson, B.A., B.Sc. 



The author described and exhibited a new polarising prism of Iceland spar so 



cut that the axis of the prism is exactly at right angles to the optical axis of the 



crystal, and that the film of Canada baLsam, by which the ordinary ray is totally 



reflected, is in a principal plane of section. 



' Published in extenso in the Joii/rnal of the Royal Microscopical Society for 1881, 

 p. 653. 



^ Printed in extenso, Phil. Mag., Nov. 1881. 



o o 2 



