56 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
The smoked glass is in the form of a wedge which is inserted be- 
tween the eve 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 its 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 terms, 
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 screens 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 photometer's. 
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? — In 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 in 
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 tke 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 means of 
monochromatic glass plates, did not lead to any precise results, they 
e The present discussion is necessarily very sketchy, because it is largely suggestive of 
possibilities rather than actual accomplishments in ecological work. For a complete 
discussion of spectroscopy and its possibilities we must refer the reader to such a mono- 
graph as Baly's "Spectroscopy" (52). 
