56 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



One of the simplest types of comparison photometers is the smoked- 

 glass type used by Wagner and credited to him by Zon and (da- 

 (78), although undoubtedly invented much before his time. The 

 smoked glass is in the form of a wedge which is inserted between 

 the eye and the source of light, until the thickness attained by 

 moving it one way or the other is just sufficient to cause complete 

 absorption of the rays. The wedge is calibrated along it- entire 

 length, and the comparison is made between the modified light under 

 study and the full, direct sunlight. Since the luminosity of the 

 latter is variable, the results are not, of course, in absolute term-, 

 even if the subjective error were eliminated. 



The Sharpe-Millar (73) photometer is a more recent development 

 and probably superior to other photometers using comparison Lamps, 

 in that the light is supplied by electric current, and the comparison 

 lamp may be standardized as often as necessary by varying the 

 amperage. Kimball (63), however, in using it through a great 

 range of daylight values, found it necessary to have screen- to cut 

 down the intensity of the daylight to the range of the artificial light; 

 also blue-grass screens to reduce the lamplight to the color of day- 

 light (skylight) alone. It is thus seen that a comparison of sun- 

 light with artificial light has various complicating factors, even 

 with the best of photometers. 



With certain correction factors arising from the use of these screens 

 the distance of the comparison lamp from the photometric device 

 is made to express directly the power of the illuminating source 

 in foot-candles. 



9. Spectroscopic measurements?'~-Yn the spectroscope the rays of 

 any light are separated according to Wave Length. This naturally 

 makes it possible to note the presence or absence of those wave- 

 lengths which are known to be essential to the plants under con- 

 sideration, so that spectroscopic observations promise much to the 

 student of ecology. Unfortunately, however, with the ordinary 

 spectroscope, observations must be ocular and confined to the visible 

 or middle portion of the spectrum. Both the highly active chemical 

 region of the ultra-violet and the strong heating rays of the infra- 

 red, are outside of observation. 



Zederbauer (79) made spectroscopic observations of the light m 

 the forest, from which he concluded that there is a marked difference 

 between the absorption by pine and spruce, or intolerant and toler- 

 ant species, respectively. The former absorb more strongly near 

 the red end of the visible spectrum; the latter more strongly in the 

 violet region. While Zederbauer's observations and his attempt to 

 reproduce the transmitted wave lengths separately by mean- of 



« The present discussion is necessarily very sketchy, because it is Largely suggestive ol iher 



ual accomplishments in ecological work. For a complete discussion of spec! n 

 sibil must, refer the reader to such a monograph as Baly's " Spectroscopj 



