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NOTES ON A METHOD OF COMPARING THE RELATIVE 
OPACITIES OF ORGANIC SUBSTANCES TO THE X RAYS. 
By ERNEST A. W. HENLEY, B.A. 
[COMMUNICATED BY PROFESSOR GEO. FRAS. FITZ GERALD, F.R.S., F.T.C.D. | 
[Read DEcEMBER 21; Received for Publication, DecemBeEr 23 ; 
Published January 21, 1899.] 
In the following experiments, the method adopted was one which 
was suggested to me by Professor FitzGerald. A piece of the 
substance under examination was cut in the form of a right-angled 
triangular prism, and placed with its base over a photographic plate 
enclosed in a light-tight bag which was separated from the wedge 
by a thin sheet of celluloid, so as to prevent any moisture reaching 
the plate. Beside this wedge-shaped piece, another of the same 
dimensions, but of different material, was similarly placed, with its 
thin edge in the same straight line with the edge of the first wedge. 
The two wedges were then photographed by the X rays. On 
examining the negative, a gradual increase in the opacity of each 
wedge from the thin end towards the thick end was observed, as 
might be expected. Several prints of varying depths of colour 
were taken from the negative, and then the actual process of 
measurement commenced. ‘The prints were cut at right angles to 
the thin edges of the wedges, and part of the photograph of one 
substance was placed in apposition with that of another sub- 
stance. Now, if the substances under examination had the same 
opacity to the X rays, then, for points equally distant from the 
thin edges of the wedges, equal depths of colour would be observed. 
Tf, on the other hand, a certain substance A was more opaque to 
the X rays than another substance B, then, to obtain equal depths 
of colouring, it would be necessary to go back farther from the 
edge of the wedge B than from the edge of A. In the case of 
wedges of the same dimensions, the thickness at any point is pro- 
portional to its distance from the thin end of the wedge, measured 
along its base line. 
