﻿a Particle from Radium in passing through Matter. 137 



a particles were projected uniformly in all directions from 

 the thin film of active matter distributed over the wire. This 

 sharpness of the edge of the photographic trace with or 

 without the magnetic field materially assisted in making an 

 accurate measurement of the distance between the centres of 

 the deflected bands obtained by reversal of the magnetic field. 



In connexion with the experiments described in this and 

 later papers, I have, in all, made more than a hundred photo- 

 graphs showing the effect of a magnetic and an electric field 

 in deflecting a pencil of a rays under various conditions. 

 Since each of these photographic traces, obtained on the 

 plates, had to be accurately measured up, it may be of 

 interest to others engaged in similar observations to describe 

 in some detail the method finally adopted. 



A microscopic method was first used, but this proved very 

 trying to the eyes, and could only be used with advantage 

 when the photographic traces were strongly marked. I 

 finally had recourse to the following procedure. The small 

 piece of photographic plate was mounted on a clear glass 

 plate of the same size as a lantern-slide. This was placed as 

 a slide in an ordinary arc-lantern and a magnified image of 

 the photograph thrown on a screen. In most of the expe- 

 riments, the magnification used was between 20 and 40, 

 and was determined by means of an accurately subdivided 

 millimetre-scale on glass placed in the position of the photo- 

 graphic plate. The image to be measured was thrown on a 

 cardboard screen fastened into a large drawing-board. The 

 observer with a sharp pencil then marked the edges of the 

 photographic traces at intervals along the whole length. The 

 screen was then removed and lines drawn through the points 

 of observation, and the distances between the parallel lines 

 measured by means of a scale and vernier. 



With a clear photograph, the measurements obtained by 

 two separate observers in the course of a few minutes did not 

 differ more than a half per cent., while the mean of a number 

 of observations was in very close agreement. 



The great advantage of the lantern over the microscopic 

 method lies in two factors. In the first place, the observer 

 sees clearly the whole outline of the photographic traces 

 before him, and is not likely to make a false estimate of the 

 edge of the line on account of local imperfections of the plate. 

 In the second place, even very weak photographic traces show 

 up clearly on the screen, and can be measured with much 

 greater rapidity and certainty and with less labour than by the 

 use of the microscope. 



