<358 Mr. L. Kolowrat on an Automatic 



One would fancy that, of the measurements given above, 

 those of d = 2Q 9, x x — ?/ 1 =42*0, ^ 2 — ^2=15*0 might be taken 

 as exact almost to a millimetre. Now the above formulae 

 lead to 



/== d-7-{x l /y 1 -d(y 1 /x l )l{d + x 2 -y fl -x 1 +y 1 )} : 



and if we take as exact the above given values of x t and y x 

 with x 2 —y2 and d, we obtain 051 for the focal length and 

 1*95 for the distance between the nodal planes ; whereas, if 

 we assume as exact the given values of x 2 and y 2 with x 1 —y 1 

 and d (in this case — 26"9, with interchange of the sub- 

 scripts in the formula), we find — 0'25 for the focal length 

 and 15*1 for the distance between the nodal planes. 



It would be interesting to know what is the exact focal 

 length, found otherwise, of the combination in question and 

 the exact reason of these most discrepant results. A method 

 requiring the determination of only one centre of rotation 

 together with two measurements that can easily be made 

 with very fair accuracy would seem a priori to be more 

 reliable than one requiring the determination of a second 

 centre of rotation as well. 



It may be noted that the point of rotation of the com- 

 bination in any position is the intersection of its axis with 

 the line joining a point and its image. 



Christ Church, Oxford, 

 Feb. 1(3, 1917. 



XXXV. On an Automatic Device for charging Electroscopes. 

 By L. Kolowrat, Assistant at the Mineralogical Labora- 

 tory, Imperial Academy of Sciences, Petrograd *. 



HAVING had to construct a set of sensitive aluminium- 

 leaf electroscopes for laboratory and field work with 

 radioactive minerals, I have fitted them with a special 

 charging device which, though somewhat elaborate in 

 design, is simple and convenient in use, and may therefore 

 deserve a few words of description. It consists essentially 

 of an ebonite rod a which, whilst being pushed down by 

 means of the button b, is rubbed against cloth placed at c, 

 e. g., in form of a pile of perforated disks. During the 

 subsequent motion, the charge thus produced is collected on 

 the fixed brass tube d, insulated by the ebonite collar e. 



* Communicated by Sir E. Rutherford. 



