Practical Photometry. By W. J. Dibdin. (Walter King.)— There is,
it seems absurd to say so, no universal standard of light and no universal photometer; innumerable suggestions have been made, and some are put into practice. Mr. Dibdin's guide, which is a sumptuously illustrated one, really fills a gap, for it is clearly written, and describes the different photometers of the day, the principle of their application, and how to ivork them. The modi- fications of modern photometry have chiefly been the product of gas illumination, and the writer explains fully the system of gas- testing as it now stands. Both the theory and the practice are discussed, so that it may fairly claim to be comprehensive ; nor are "colour photometry" and "stellar photometry" neglected, the author having devised a chromatic standard himself. This is a capital book, and deserves to be a success.
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