Abstract
DR. HOUSTOUN, referring in NATURE of January 30, p. 159, to Dr. Toy's note on the above, deprecates, the use of ‘neutral’ absorbing screens, and recommends that the intensity of a beam of radiation be reduced by increasing the distance between source and screen and applying the inverse-square law. In view of the simplicity of action and ease of calibration of neutral screens or wedges, it is surprising that at this stage in the development of photometry, a method possessing so many disadvantages as that suggested by Dr. Houstoun should be seriously considered. If large variations in the intensity of radiation are required, say of the order of 2000 to 1, any instrument depending on the direct application of the inverse-square law must necessarily be extremely cumbrous. Further, the effect of reflected radiation from adjacent surfaces is difficult to eliminate, and the necessary correction for the absorption of the intervening medium detracts from the apparent simplicity of the method.
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