=76. Methods.= All methods for measuring light intensity, which have been at all satisfactory, are based upon the fact that silver salts blacken in the light. The first photographic method was proposed by Bunsen and Roscoe in 1862; this has been taken up by Wiesner and variously modified. After considerable experiment by the writer, however, it seemed desirable to abandon all methods which require the use of “normal paper” and “normal black” and to develop a simpler one. As space is lacking for a satisfactory discussion of the Bunsen-Roscoe-Wiesner methods, the reader is referred to the works cited below.[4] Simple photometers for making light readings simultaneously or in series were constructed in 1900, and have been in constant use since that time. An automatic instrument capable of making accurate continuous records proved to be a more difficult problem. A sunshine recorder was ultimately found which yields valuable results, and very recently a recording photometer which promises to be perfectly satisfactory has been devised. Since the hourly and daily variations of sunlight in the same habitat are relatively small, automatic photometers are perhaps a convenience rather than a necessity. _The Photometer_ [Illustration: Fig. 11. Photometer, showing front and side view.] =77. Construction.= The simple form of photometer shown in the illustration is a light-tight metal box with a central wheel upon which a strip of photographic paper is fastened. This wheel is revolved by the thumbscrew past an opening 6 mm. square which is closed by means of a slide working closely between two flanges. At the edge of the opening, and beneath the slide is a hollow for the reception of a permanent light standard. The disk of the thumbscrew is graduated into twenty-five parts, and these are numbered. A line just beneath the opening coincides with the successive lines on the disk, and indicates the number of the exposure. The wheel contains twenty-five hollows in which the click works, thus moving each exposure just beyond the opening. The metal case is made in two parts, so that the bottom may be readily removed, and the photographic strip placed in position. The water-photometer is similar except that the opening is always covered with a transparent strip and the whole instrument is water-tight. These instruments have been made especially for measuring light by the C. H. Stoelting Co., 31 W. Randolph street, Chicago, Ill. The price is $5.