214 MR. 8. LE M. MOORE’S STUDIES 
means of a venetian blind, except for a narrow space below. 
Under these circumstances there can be no doubt that the 
photrum was curtailed at the positive end. Then with regard to 
the end abutting on darkness (hereafter referred to as the 
“negative” end), the diminishing intensities of light were ob- 
tained by the use of screens, carefully adjusted it is true, yet 
necessarily leaving much to be desired on the score of accuracy : 
here, also, it is probable that the photrum was too much shortened. 
Another objection is that the quality of sunlight is very variable, . 
depending, as it does, upon the amount of condensed and aqueous 
vapour in the atmosphere; and this variation must obviously 
extend over the whole photrum. Results obtained by this 
method are therefore strictly comparable only in respect of ex- 
periments made on the same day and between the same hours. 
The foregoing considerations render necessary the proviso that 
the statements which immediately follow represent the facts 
_ Fig. 1. + 
. Elodea canadensis. 
. Lemna trisulca. 
. Saxifraga granulata (position of positive critical point). 
. Oxalis Acetosella (position of negative critical point not exactly determined). 
. Pteris cretica (positive critical point). 
. Scale of garden Chrysanthemum (Pyrethrum sinense). 
(In this and the following diagrams the epistrophic interval is shaded.) 
D Oe UN 
merely in a rough-and-ready way, which nevertheless will, per- 
haps, be deemed preferable to a simple unillustrated account. 
The first diagram (fig. 1) was constructed on a fine day in No- 
vember. In this, as in the following experiments, the photrum 
having been duly adjusted, the leaves to be experimented with 
were rapidly cut off and mounted in water upon ordinary glass 
nS 
