472 Scientific Proceedings, Royal Dublin Society. 



of tissue]. In curve C the vertical scale is 10 times as open, i.e. the activity 

 of the bare tube is now 1000. The horizontal scale is normal. Tlie use of 

 these varying scales is rendered necessary by the great range of activities 

 encountered with light screens. 



Curves are also shown for aluminium, brass, .and lead. In each case the 

 scales are the same as for curve C, the activity of the bare tube being 1000, 

 and the horizontal unit one gram per sq. cm. In using these curves the 

 density of animal tissue may be taken as TO, that of aluminium as 2-7, 

 brass 8-0, and lead 11-3. Thus 1 mm. of brass corresponds to O'S gram per 

 sq. cm., and so on. 



The following points may be noted : — 



(1) The stopping power of a given mass of the screen for /3 rays increases 

 with its atomic weight. 



(2) The primary f^ rays are completely absorbed by 14 mm. of tissue, 

 5 mm. of aluminium, 1'5 mm. of brass, or 1 mm. of lead. These figures are 

 obtained by dividing the abscissae for which the various curves become 

 approximately horizontal by the densities of the corresponding screens. 



(.3) Owing to differences in secondary radiation, the total ionisation is 

 not identical for various screens of just sufficient thickness to stop the 

 primary j3 rays in each case. The peculiar crossing of the brass and lead 

 curves is to be attributed to this cause, as the latter is raised by the 

 large secondary radiation from the lead. It subsequently falls below the 

 former again, owing to the greater absorption of y rays by lead. 



Greater thicknesses of leather could not be tested with the arrangement 

 employed ; but this is not a serious disadvantage, as in the calculations which 

 follow the comparatively unimpoi'tant absorption of 7 rays at such depths 

 can be estimated with sufficient accuracy fiom the results obtained for brass, 

 or taken from the figures obtained by other workers. 



Tests were also made with an emanation tube enclosed in a standard 

 serum needle 1'2 mm. external diameter, and various card and leather 

 screens. These showed that the stopping power of the needle (apparently 

 about O'l mm. thick) was very nearly equivalent to 1 mm. of tissue. This is 

 used below. 



Effect of Distance. Capillary in Needle. 



The effect in the tissues at various distances from a needle containing 

 an emanation tube has been calculated, and is shown in column A of the 

 table on p. 475, in which a represents the distance from the centre of the 

 needle. In making this calculation the emanation, instead of being distri- 

 buted throughout a tube 15 mm. long by 0'6 mm. internal diameter, is 



