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 difificult problem. A sunshine recorder was ulti- 

 mately found which yields valuable results, and very recently a recording 

 photorneter which promises to be perfectly satisfactory has been devised. 

 Since the hourly and daily variations of simlight in the same habitat are 

 relatively small, automatic photometers are perhaps a convenience rather than 

 a necessity. l[jj.yvtAA^^ 



-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 van Wien, Cairo, und Buitenzorg, 



(Java) Denksch. Kajs. Akad. Wien., 64. 1896. 

 Untersuchungen iiber den Lichtgenuss der Pflanzen im arktischen Gebiete. Sitzb. 



Kais. Akad. Wien., 109. 1900. 



