﻿838 Prof. R. P. Salmi on the 



considered to be the most convenient, a development of 

 about 2\ minutes was the best. For lower temperatures, 

 this period was much longer. The other plates made by 

 the same firm gave unsatisfactory results. The Ilford 

 Process and the Imperial Sovereign plates come next after 

 the Wratten lantern-plates. The following developer was 

 found most suitable to be used with the plate : — 



Solution A. 



1. Quinol, 25 gm. 



2. Potassium metabisulphite, 25 gm. 



3. Potassium bromide, 25 gm. 



4. Distilled water made up to 1000 c.c. 



Solution B. 



1. KOH, 50 /rm. 



2. Distilled water up to 1000 c.c. 



Solutions A and B to be mixed in equal proportions 

 immediately before use. 



A 20 per cent, solution of " Hypo "" was used for fixing. 



It is of importance to exclude even red light as much as 

 possible during all the operations. It is also important to 

 use water as free as possible from dust nuclei. For this 

 purpose the water for washing should be filtered or previ- 

 ously boiled and cooled. 



Some experiments with oc particles. — In the experiments 

 here referred to, both with a and j3 particles, the active 

 sources employed consisted of polonium (containing some 

 radium E), thorium active deposit, and radium C on the 

 tip of fine sewing-needles and on suitable metal plates of 

 different dimensions. In the case of the needles, only the 

 tip was effective, the rays from the rest of a needle, if acti- 

 vated, being screened off by means of a small cardboard disk. 



The microphotograph in Plate XII. fig. 1 was obtained 

 with a very weak thorium active deposit needle, the tip 

 being exposed for a few seconds to the sensitive plate. 

 Only a portion of the microphotograph is here shown. The 

 centre is about the point marked C. Fig. 2 (a, b, and c) was 

 obtained with a fairly strong polonium needle. Fig. 3 is a 

 portion of a similar microphotograph obtained with a thorium 

 active deposit needle. In all these cases the activated tip 

 of the needle was held in contact with the photographic 

 plate. Similar results were obtained with other «-rav 

 sources. It will be noticed, at once, that a marked feature 

 of all a-ray photographs is the presence of straight tracks 

 radiating from a common centre, which is either the point 



