810 Sir E. Rutherford and Dr. J. Chad wick on the 



examination. In this way we have obtained definite proof 

 that, in addition to nitrogen, also boron, fluorine, sodium, 

 aluminium, and phosphorus exposed to u rays give rise to 

 particles whose ranges vary between 40 and 90 cm. of air. 



In the present paper, a brief account will be given of the 

 general methods employed in examining a number of 

 elements and also a more detailed account oi: the variation in 

 the number of particles with velocity of the incident « rays 

 for two typical elements, viz., nitrogen and aluminium. 



Counting of Scintillations. 



With the microscope originally employed, the scintillations 

 due to H atoms were in general weak and difficult to count 

 with accuracy. The brightness of the scintillations can be 

 much increased by the use of special lenses. We have found 

 most suitable for our purpose a holoscopic objective of focal 

 length 16 mm. and aperture *45. With an eyepiece of 

 low magnifying power, this gave a field of view of diameter 

 3*25 mm. and of area 8'3 sq. mm. The magnification of the 

 system employed w r as about 40. In these experiments it 

 w^as essential to include as large an area of screen as possible 

 in order to give a convenient number of scintillations per 

 minute. For this reason, the use of short focus objectives, 

 which can readily be obtained of wide aperture, was quite 

 out of the question. In order to avoid the direct effect on 

 the eyes due to the 7 rays from the source of radiation, a 

 special microscope was constructed in which the light from 

 the objective was turned through a right angle by means of 

 a totally reflecting prism. In this wa} r , by use of suitable 

 absorbing screens, the observer could be protected from the 

 direct effect of the powerful sources of 7 rays employed in 

 some experiments. This microscope was designed and con- 

 structed for the purpose by Mr. Twyman of Hilger and Co., 

 and has proved very satisfactory. 



An account of the precautions to be taken in counting- 

 weak scintillations has been given in a previous paper, but 

 attention should be drawn to a few r additional points. In 

 order to reduce the luminosity of the zinc-sulphide screen 

 due to the 7 rays, it is important to employ a thin and finely 

 powdered layer. With the use of a strong magnetic field to 

 turn aside the /3 rays, we have found it feasible to count the 

 scintillations with a source of radium C of activity equivalent 

 to 20 mgs. Ra only 2*5 cm. away from the zinc-sulphide 

 screen. With the new microscopes we have found the 

 counting results much more concordant than w r ith the old, 



