424 BOTANICAL GAZETTE [DECEMBER 
for their records upon the absorption of incident radiations, it is 
essential that the angle of incidence of the impinging light be always 
the same. But the direction of incidence of the sun’s rays is 
constantly changing throughout the day, and varies, for the same 
hour, from day to day. It is therefore necessary to consider this 
matter in the operation of any and all forms of absorbing instru- 
ments. The Hicks instrument cannot be adjusted in this regard, 
for the main absorbing surface is the meniscus of alcohol in the 
upper bulb. 
The porous cup atniometer possesses a cylindrical absorbing 
surface, modified slightly at the closed end of the cup, which latter 
part may readily be removed from operation by a suitable covering 
if desired. Iam convinced that the error involved from the curved 
end of the cup, however, is negligible in this sort of estimation. 
The form of cup used was the usual one, the modification recently 
described by Transeau (Bot. Gaz. 49:459. 1910) would no 
doubt be as efficient for our purpose. To receive the sunlight 
always at the same angle, the cups are so placed that their long 
axis is perpendicular to the direction of light incidence at noon, 
the common plane of sun and cylinder being vertical. When so 
arranged the sun virtually rotates about the cup, its rays always 
illuminating one-half of the surface only, and falling always verti- 
cally upon a longitudinal line through the center of the lighted 
area. The position of the lighted area on the cup is constantly 
changing, but since all sides of the cup are supposedly equivalent, 
this introduces no complication. The position of the instrument 
will of course vary with the sun’s altitude, that is, with latitude and 
season, but may readily be determined from an almanac or by simple 
observation at high noon. Actual tests showed clearly that the 
vertical cup, as ordinarily used, fails to record proper intensities 
of sunshine at and about noon, for at that time only a small portion 
near the tip receives perpendicular radiation. Of course in high 
latitudes the vertical position would not introduce so great an error 
as nearer the equator, and the error in winter would be less than in 
summer. The black bulb thermometer is to be exposed in the same 
way as the cups. 
The photographic papers were always exposed by hand, so placed 
