296 Sir Jamas Mmksiizie Davidson [Feb. 2, 



Diagram 2, showing how these rays may produce 

 photographs. 



Pinhole photographs. Photographs of different 

 layers of tinfoil by means of the primary and secondary 

 rays. 



Show electroscope, and describe by means of which 

 a comparison of the primary and secondary rays has been 

 made.] 



Protection. — Many years ago I made an experiment in which a 

 Crookes tube was completely buried in a large quantity of red lead 

 contained in a box, and when this was excited in a dark room the 

 fluorescent screen showed that no a;-rays at all were able to penetrate. 

 Then a small opening was made, by scooping away the red lead until 

 the primary rays got through, together with only a very few of the 

 secondary rays from the small area of glass opposite the orifice. This 

 was the most effective means of screening everybody from the rays 

 except the individual under observation, but it was highly incon- 

 venient, the apparatus being difficult to handle, especially when a 

 fresh tube had to be embedded. Therefore a box was constructed, 

 lined with a mixture of red and white lead, and a small hole was cut 

 in it for the emission of the rays. Some such method is now fairly 

 generally adopted, and is most important for the protection of the 

 workers. 



When a fluorescent screen is used for examining patients, it should 

 be lined with thick lead -glass. This allows the shadows of the screen 

 to be perfectly observed, while at the same time protecting the faces 

 of the observers from the injurious effects due to the a;-rays. Since 

 these precautions have been taken the disasters to workers have 

 been minimised, and should be entirely abolished. The opacity 

 to the «-rays of any material is in proportion to its atomic weight. 

 Therefore any material impregnated with a compound having heavy 

 atomic weight would act as an effective protection. 



Pastilles. — Great difficulty was experienced at first in the use 

 of a;-rays owing to the varialjility of the tube. No reliable means 

 existed of knowing what dose — to borrow a term from medicine — was 

 being given. At present we have several fairly reliable methods 

 which have led to more precise results without bringing about the 

 unfortunate injuries previously occurring. The method most in 

 general use is that of the Sabouraud pastille, which consists of barium 

 platino-cyanide in compressed form — the same material as that which 

 is employed in the making of the fluorescent screen. 



The pastille changes gradually in tint from a pale yellow to a deep 

 orange, according to the amount of a;-radiation it receives. It is 

 arranged in a definite position, and by careful experiment sufficient 

 data have been forthcoming to enable any desired biological result to 

 be obtained with approximate precision. The great advantage of 



