Photometrical Measurements

Abstract

AS a “manual for the general practice of photometry, with especial reference to the photometry of arc and incandescent lamps,” this work will be found useful. Most of the descriptions of photometers are clear and well illustrated, and much practical information about standards of light is collected together. That strange medley of apparatus enshrinedin an expensive tabernacle of mahogany and velvet called by gas engineers a “photometer” is not even mentioned, possibly because the book is of American origin. Photometers, and those parts of the art of photometry which are of use to engineers, may be defined without much difficulty, and the apparatus and methods suitable for the research laboratory may be grouped together; when to these is added the theory of the subject, the whole ground of photometry is covered. But the author makes no such distinctions, and the value of his work suffers. While his reference to spectro-photometry is meagre, and the bolometer is dismissed in less than six lines, he drags in double integration to determine the mean spherical intensity of a purely academical case of distribution. On the other hand, he treats possible cases of distribution in a clear and practical way. The description of a Bunsen photometer in the crude form of a screen without mirrors or prisms, and an ancient algebraical theory of the Bunsen screen, containing no reference to the angle of emission or direction of view, marks the author, as do many other passages, as a science teacher. He is in good company; there is hardly a text-book of physics in English in which that useless affair is not represented as a Bunsen photometer. In common with most science teachers, he assumes that the shadows of a Rumford photometer must be widely separated, and he very properly alludes to the lack of sensitiveness which results. When Lord Kelvin said that no one could need a better photometer than a pencil and a white card he knew that the edges of the shadows should meet, and, it may be added, that the shadows should completely cover the card. The little-known, but valuable, Conroy, Ritchie and Thompson photometers, varieties of the Rumford, are described, and the somewhat over-rated Lummer-Brodhun apparatus is criticised. To describe the use of the rotating sector without allusion to Abney, the light of the arc without reference to Flemings S. P. Thompson or Mrs. Ayrton, and measurement of the mean spherical candle power of arcs without reference to Blondel, can hardly be excused by the attempt to compress the whole book into 261 pages. That the author is a professor of engineering may account for the excellence of the practical parts of the manual; but hat, being a professor, some of the theoretical parts are so obscure is strange.

Photometrical Measurements.

By W. M. Stine, Williamson Professor of Engineering, Swarthmore College. Pp. xi + 270. Illustrated. (New York: The Macmillan Company. London: Macmillan and Co. Ltd., 1900.) Price 6s. 6d. net.