LIGHT 49 
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 ulti- 
mately 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 
77. Construction. The simple form of photometer shown in the illustra- 
tion is a light-tight metal box with a central wheel upon which a strip of 
Fig. 11. Photometer, showing front and side view. 
photographic paper is fastened. This wheel is revolved by -the thumb- 
screw 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 stand- 
ard. The disk of the thumbscrew is graduated into twenty-five parts, 
and these are numbered. A line just beneath the opening coincides with 
Untersuchungen iiber das photochemische Klima von Wien, Cairo, und Buitenzorg 
(Java) Denksch. Kais. Akad. Wien., 64. 1896. ; : 
Untersuchungen iiber den Lichtgenuss der Pflanzen im arktischen Gebiete. Sitzb. 
Kais. Akad. Wien., 109. 1900. 
