AUGUST I, 1912] 
NATURE 
Be) 
Exp. 2.—Set up as for experiment 1, except that | 
the 50-cp. lamp is not used. In this case, when the 
100-cp. lamp is switched off, the outer rings appear 
first and the central disc last. 
Exp. 3.—On a piece of very dark grey paper (or a 
well-used blackboard) about 36 in. square fasten a 
piece of black velvet about 9 in. sauare. Reduce the 
light in the room so that the velvet can only just be 
distinguished from a distance of about 2 ft. Gaze 
steadily at the centre of the velvet, and after about 
four seconds the outer edges of the paper will appear 
to darken. This dazkness will slowly progress until 
the paper and velvet appear to be enveloped in an 
absolutely black curtain. It is interesting to note 
that in each of these experiments the slightest move- 
ment of the eyes or eyelids is sufficient to restore 
normal conditions. So far as I am able to ascer- 
tain, these three experiments have not previously 
been recorded. They appear to indicate that the 
retina depends for its action upon sensitisation from 
the periphery inwards; also that this sensitisation is 
dependent upon light falling upon the periphery. 
Exp. 4.—Take an oculist’s ordinary test-type and 
hang it on the wali of the room fitted with the three 
lamps previously described. Choosing a T in the 
6/18 line, gaze at it steadily from a distance of about 
12 in. After a few seconds the white paper immedi- 
ately surrounding the letter appears to increase in 
brightness. At this point have the t1oo-cp. lamp 
switched off. This brightness will now increase and 
spread towards the terminals of the letter. At the 
same time the brightness takes a slightly blue tinge 
until, in the words of a friend who tried the experi- 
ment, “the letter appears to float in a Bunsen flame.” 
I should be glad if some of your readers will repeat 
these experiments, or if they have been recorded else- 
where give the reference. HERBERT S. RYLAND. 
9 Vere Street, W., July 2. 
Photosynthesis and Stomatal Aperture. 
Ix your issue of August 10, 1911, you were good 
enough to publish a brief description of my “ Stomato- 
graph” (Proc. Roy. Soc., B., vol. Ixxxv., p. 33). 1 
there pointed out that the stomatal aperture in 
Egyptian cotton plants under field conditions during 
June reaches its maximum at about 9 a.m., and that 
this maximum aperture is maintained for only a few 
hours at most, closure ensuing as the result of the 
severe water-strain on the root-system. Thus the 
stomata may be almost completely closed by noon, 
or even earlier. 
It was highly probable that this closure would be 
found to provide a limiting factor on photo-synthesis 
by restricting the inward diffusion of carbon dioxide. 
It has, moreover, been noted by Thoday (Proc. Roy. 
Soc., B., 82) that such limitation may probably 
account for the low values obtained by some workers, 
and possibly for the fact, pointed out by Blackman, 
that the theoretical possibilities of carbon-dioxide 
assimilation have never even been approached. Since 
the stomata in Egyptian cotton plants are gaping 
wide, under intense illumination, and at high tem- 
peratures, for an hour or two in the morning, there 
was a further- expectation that record values might 
be obtained. Both these expectations have been ful- 
filled. 
The error from asymmetry of the leaves is high. 
Using the Sachs-Thoday stamping method, the P.E. 
on twenty identical pairs, each 15 cm.? in area, 
worked out at +4 per cent. of the mean dry-weight. 
The dry-weight of a square decimetre of these stamped 
areas is about 0°7 grams, so that with ten pairs of 
such areas we still have a P.E. of 9 mg. 
NO. voL. 89] 
BOD 
<<}; 
In the effort to avoid this error I carried the number 
of pairs as high as 73 in a single experiment, which 
covered 16 hours, centred on 9°43 a.m., and gave an 
increase in dry-weight—without correction for trans- 
location, if any—of 22°8 mg. per square decimetre 
per hour, with a P.E. of 3 mg. The mean shade 
temperature was about 28° C. only. i 
Results of greater interest were obtained by series 
of hourly determinations, which in two cases were 
successfully carried from 8 and 7 a.m. to 6 p.m. Ten 
pairs, each 15 cm.* in area, were employed in the first 
set, and twenty, each 10 cm.” in area, in the second 
set. The results are as follows :— 
7-8 a.m. 8-9 g-10 10-11 11-12 12-1 I-2 
(1) +124 +2273 +26°8 +207 + 73 - 68 +4 83 
(2) — +237 +399 +297 +1374 +25:9 +21°5 
2-3 3-4 4-5 5-6 p.m. 
Gta 7k Om ee 507120 7a 
(ysis 28:25 5 876 
It will be noticed that in the first set the sustained 
rate from 8 to 11 a.m. works out at 23 mg. (P.E. 
5 mg.), while in the second it amounts to 25°77 mg. 
(P.E. 23 mg.) from 8 a.m. to 2 p.m. It seems quite 
certain that values of 25 mg. per square decimetre per 
hour are attained by cotton plants in Egypt; these 
values are 25 per cent. higher than have formerly 
been recorded, with known probable error. 
Turning to the effect of stomatal closure, it is 
quite clear that assimilation is very greatly reduced, 
if not inhibited entirely during the afternoon, by this 
closure. The data quoted above do not plot out to a 
smooth curve, partly on account of the high asym- 
metry of the leaf, and partly from the idiosyncrasies 
of individual plants, although the latter error was 
reduced in the second series by using twenty different 
plants, instead of three or four, for each hourly group. 
Comparing these curves, such as they are, with 
the records from stomatograph and thermograph, we 
find that assimilation seems to be limited by tempera- 
ture until about 9 a.m., and then by stomatal aperture 
for the rest of the day. Even when the stomata are 
widest, the intake of carbon dioxide is not sufficient 
to follow the temperature up to its maximum of 35° 
to 42° C.; during the afternoon the plant is starving. 
W. Lawrence Batts. 
Gezira House, Cairo, July 3. 
Curie’s Constant in the Ferromagnetic State. 
In reference to my brief letter on this subject ap- 
pearing on July 18, I should like to say that while 
the relative values of Curie’s constants for iron and 
nickel agree with those of the analogous constants 
in the ferromagnetic state, the absolute values in the 
two states are connected by a factor of the order of 
1o°, and the constants are only independent of the 
temperature each for its own state. 
July 20. J. R. AsHworrtu. 
Elliptic Functions. 
I VENTURE to appeal for information as to tables of 
elliptic functions of the second kind, those by which 
the lengths of elliptic arcs are evaluated. The best 
tables accessible to me are those in Dale’s very useful 
book, ‘* Five-figure Mathematical Tables,’’ and these 
are too brief for my purpose. 
I have failed to obtain Legendre’s original tables. 
But surely these have been reprinted, either in full, or 
in a shape more detailed than that I have mentioned. 
Possibly there may be a French or a German edition, 
failing an English one. I shall be grateful for any 
definite information. C. T. WHITMELL. 
Hyde Park, Leeds, July 27. 
