1907] BRIEFER ARTICLES 277 
rubber tubes are slipped off under water, allowing the mercury to run out, 
and are then used as a reagent tube, the reservoir being stoppered for the 
pu . 
When plant parts in which the respiratory ratio is not 0 are studied, a 
preliminary test is made to ascertain in which direction the ratio varies, 
and then the starting-level can be adjusted accordingly. 
The usual corrections of course must be made. The gas pressure 
inside at the.time of reading is equalized with the atmospheric pressure 
by sliding the reservoir tube up or down until the levels inside and out are 
equal. For very exact work it would be necessary to take account of the 
barometric pressures, but the slight error of this sort is negligible in demon- 
stration. The temperature must either be made the same at the start of 
the experiment and the final reading; or else, as is readily possible, the 
change of volume due thereto must be calculated. Vapor tension should 
also be considered in exact work, but it is negligible in demonstration. 
After each use the instrument should be thoroughly washed clear of potash. 
IX. NORMAL LIGHT SCREENS 
In several phases of the study of photosynthesis it is necessary to apply 
some form of light or color screen to a leaf. To this end various arrange- 
ments are extemporized which, however inconvenient and time-consuming 
to prepare, are scientifically unexceptionable so long as they are physiologi- 
cally accurate. But unfortunately some of these devices are logically falla- 
cious and yield results which are only accidentally correct. This is true, 
for example, of that experiment common in elementary demonstration, 
where disks of cork, tin-foil, etc., are placed matching on opposite sides of 
the leaf in order to exclude light; in fact, they also exclude the equally 
indispensable carbon dioxid. 
Screens correct in principle must allow for free access and exit of gases, 
but in their construction advantage may be taken of the fact that in ordinary 
leaves the stomata are either largely or wholly on the under surface, so that 
if this is left free the upper surface may be covered as closely as desired. _ It 
is upon this principle the two forms figured herewith (fig. 2) are constructed. 
The larger, designed to take an entire leaf of moderate size, consists of a 
wooden box readily adjustable for height and angle, 57X14 inches 
(internal), white without and black within, separated lengthwise into two 
compartments with an intermediate space for petiole and midrib. e 
bottoms of the compartments are largely open, but so matched by dia- 
phragms that air can enter freely but no direct light. Movable gratings 
of silk threads hold the leaf firmly but elastically against the glass cover, 
