Third. Is it an unconscious adaptation to surroundings, such as other organs undergo under changes of environment? [Illustration: PLATE XI SPECIFIC COLOUR CURVES OF HEALTHY AND DISEASED HUMAN BLOOD _To face page 82._] [_Lovibond, Colour Theories._ ] Appendix III THE APPLICATION OF THE NATURAL LAW OF SPECIFIC COLOUR RATE BY DR. DUDLEY CORBETT TO THE EXACT MEASUREMENT OF X-RAY DOSAGE. Dr. Dudley Corbett. The gradations in the tint given by the Sabouraud-Noiré pastille when exposed to X-rays are so fine, especially in that region of the colour scale where lies the erythema dose, that many have felt the want of a more accurate means of reading these tints, as well as a series of reliable standards for comparison. Hitherto the only methods at all generally used have been Hampson’s radiometer and Bordier’s radio-chronometer, the former in this country, and the latter on the Continent. Hampson’s instrument has two disadvantages: it can only be used with electric light, and the standards are made of tinted paper liable to get soiled, and to vary slightly with the changes in the pigment employed. Its advantage is that it may be used as a sliding scale, thus economising the pastilles. Some persons, however, have considerable difficulty in reading the tints, the scale rising only by gradations of 1/4 B. In the construction of any such instrument, the really important point is to obtain a reliable standard for Tint B--_i.e._, the normal epilation dose. The tints on the Sabouraud card itself are not always identical, some representing a dose which will only just epilate, others an almost dangerous dose for unfiltered rays. The Tint B, which is the standard, allows a margin of error of 20 per cent. on either side. In other words, 4/5 B will almost always epilate, while 1-1/5 B is nearly the limit of safety. This observation is in accordance with the experience of other workers on this subject. In my instrument Tint B has been obtained by measuring the pastille with Lovibond’s tintometer immediately after exposure to the X-rays. The pastille was turned to a tint corresponding to an epilation dose which was known to be safe, as proved by clinical results. This tint was measured directly both by daylight and by artificial light from an 8-candle power carbon filament lamp with frosted glass shade. I am indebted to Mr. Dean for suggesting the use of Lovibond’s instrument for this purpose.