Abstract
IN the course of some experiments on Poisson's ratio by Cornu's method1, one of us (B. J. G.) used a photographic method of recording the observed fringes for sodium light. Ilford Thin Film Half Tone Panchromatic plates were used with Johnson's Contrast Developer, and it was noticed that the resulting pattern showed very fine white fringes on a black ground in the negative; comparison with visual observation showed that the recorded fringes were much sharper. It was concluded that this was due to the very high contrast of the photographic process used, and it appeared desirable to make a fuller investigation of this phenomenon. The use of high contrast is well known in the photography of subjects of low intrinsic contrast; but the possibility that variation of exposure would produce markedly different negatives seems to be little known and certainly is not widely used.
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