[Proc. Roy. Soc. Victoria, 31 (N.S.), Part II., 1919]. 

 Art. XX. — The Sensitiveness of Photographic Plates to X-rays, 



BY 



NATALIE C. B. ALLEN, B.Sc, 



AND 



T. H. LABY, M.A. 



(Professor of Natural Philosophy in the University of MelViouine). 



(With Four Figures in the Text). 

 [Read 12th December, 1918]. 



This work was undertaken in order to determine the relation 

 existing between the exposure and the resulting optical density of 

 a plate exposed to X-radiation, and, the type of plate most suitable 

 for radiographic w^ork. On the first question (the relation be- 

 tween exposure and density) work had previously been done by 

 Salomonsoni and by Hodgson. 2 



Ihe plates under investigation were given a known exposure^ 

 developed in a standard manner and their' densities were measured 

 by a polarisation photometer. 



For generating the X-rays a Coolidge tube excited by an induc- 

 tion coil was used, the potential across the tube being regulated 

 by adjusting the length of the alternative spark-gap of the coil 

 until a spark passed occasionally. 



It was assumed that the exposure was proportional jointly to the 

 energy of the X-radiation and to the time of exposure. The cor- 

 responding law for light is the Bunsen-Roscoe reciprocity law that, 

 for equal densities of the photographic plate, the product I.t of 

 intensity of light and time of exposure is constant.' This law is 

 not correct, and has been replaced by the Schwarzschild law that 

 LtP is constant for equal densities. Ihe value of the constant p 

 depends on (1) the kind of plate used, (2) the development," (3) the 

 density of the plate, and (4) the colour of the light used.* Stark^ 

 proposed the formula that k.I.n^tn is constant for equal densities, 

 where^k, m, n are constant for small variations of I. and t. Kron^ 



1 Salomonson. Proo. Anisferflain Acad, xviii . 191({, p. 671. 



2 Ho(l;;^son. Anier. Joini). Roeiit, 1017, p. filO. 



3 Koch. Ann. d. Phys , iv., 30, 1909, p. 841. 



4 Paikhurst. A-strophysikal. .Jour.. .SO, }9t)9. 

 .5 Stark. Ann. d. Phys., iv., :r), 1911, p. 4«'. 



e Kroji. Ptiblikatiorit'ti des Astrophysikali-schcii Ol>servatoriim)8 zu Potsdam, No. (J7, 1913. 



