210 
NATURE 
[May 2, 1612 
so-called rest state) to greater. It is not a measur- 
able physical change, but a vital one which cannot 
be measured, and which therefore lies outside the 
scope of scientific knowledge. The two acts which 
seem to constitute the excitatory process, viz. excita- 
tion and response, are not continuous, but are joined 
together by a non-measurable link. This link is a 
subject of scientific conjecture, not of scientific know- 
ledge; for nothing that is not measurable is known. 
It is, in short, something which is involved in 
organism, for which the most proper designation is 
organismal, 
“The point to be emphasised is that the organismal 
link or nexus is the essential part of the excitatory 
process; for neither the physical effect of the stimulus 
nor that of the response is effectual by itself. It is 
only when these two are coupled by the organismal 
nexus that the excitatory process is constituted. 
“The propagation of the excitatory process thus 
constituted tales place, not through or by any 
measurable process, but is wholly and solely 
organismal, and therefore not measurable. The 
electrical machinulee are acted on by the organismal 
stuff, and not by their neighbours. Propagation is 
a vital process, not a physical one. 
“The purpose of the paper will be (in case it is 
ever written) to show (1) that the mere statement of 
measurable data stops short of its purpose because it 
misses the essential fact in the excitatory process; 
(2) that every electrical change accompanying excita- 
tion which is cyclical corresponds to a single organ- 
ismal change; (3) that the organismal change is 
modified by (a) exhaustion and (b) injury, these 
being localised (a) at the proximal contact and (b) 
at the distal, and having opposite signs.” (Here, 
of course, the ligatured muscle preparation is re- 
ferred to.) 
While your reviewer is certainly right in empha- 
sising, as, indeed, we have done in the Memoir, 
Burdon Sanderson’s strong objections to vitalism, it 
seems to me that in this letter he also lays his finger, 
deliberately and accurately, on the weal spot in the 
physico-chemical theory of life. It is the connection 
between ‘‘physical’’ or ‘‘chemical” stimulus and 
“ physiological’? response that is unintelligible from 
the point of view of the physico-chemical theory of 
life. Burdon Sanderson concluded that this connec- 
tion lies outside the scope of scientific knowledge, 
and in this way he avoided the many scientific diffi- 
culties and defects of traditional vitalism. But we 
felt bound to point out the gap which is left if no 
attempt is made to deal scientifically with what he 
calls (the italics are his own) “the essential part of 
the excitatory process.” In every department of 
physiology there is the same gap; and what remains 
for exact physical and chemical investigation would 
seem to be only the outer fringe of the real pheno- 
mena. . J. S. Harpane. 
Oxford, April 16. 
A Peculiarity in the Shadows Observed during a 
Partial Eclipse of the Sun. 
Durinc the recent partial eclipse of the sun, I 
observed a peculiarity in the shadows cast by the 
sun’s rays which may be worth recording. If eclipses 
occurrea more frequently, this peculiarity would be 
familiar to everyone; as it is, I am not aware that it 
has been noticed, which is probably due to the fact 
that the attention of most observers was concentrated 
on the appearance of the sun itself. : 
While the eclipse was progressing I was walking 
along a country road white with dust. Along the 
edge of the road were young trees about 15 ft. high, 
decked with small, undeveloped leaves. The shadows 
NO. 2218, VOL. 89| 
of these trees, cast on the road, presented a peculiar 
appearance. What first attracted my attention was 
the number of salient angles in the shadows: these 
angles were not due to the shapes of the leaves, 
which were practically oval. A closer scrutiny re- 
vealed the fact that, where a leaf was isolated, its 
shadow took the form of a crescent; in fact, each 
such shadow was a negative image of the visible 
portion of the sun’s disc. 
This observation recalled to my mind a _ pheno- 
menon to which my attention was directed some 
years ago by Mr. L. H. Winn. Mr. Winn observed 
that if a white screen be placed at some distance 
from a window which looks towards the sky, and a 
pencil be placed between the window and the screen, 
the shadow of the pencil takes the form of a faint 
negative image of the window, the vertical sashes 
being represented by bright lines, while the clear 
panes are represented by dark rectangles. Mr. Winn 
explained this phenomenon correctly by tracing the 
paths of individual rays; he also performed a number 
of other experiments which confirmed his reasoning. 
The explanation which will be given in this communi- 
cation is of a somewhat more general character, and 
is a particular instance of Babinet’s principle. The 
following laboratory experiments illustrate the pheno- 
menon to be explained in a striking manner, and, at 
the same time, suggest its explanation. 
Let a magic lantern (preferably illuminated with an 
arc lamp) be directed towards a white screen at a 
distance of 15 to 20 ft. Remove the focussing lens 
and cover the condenser with tissue paper. Next, let 
the circular illuminated area of the tissue paper be 
partly covered with an opaque disc, so that a brightly 
illuminated crescent remains visible. Place a sheet 
of cardboard, in which a hole about a centimetre in 
diameter has been bored, between the lantern and 
the screen. An inverted positive image of the illumin- 
ated crescent is formed on the screen: this image is 
produced in accordance with the principles which are 
exemplified by the pinhole camera. When the per- 
forated sheet of cardboard is removed, the illumina- 
tion of the screen is approximately uniform; and if 
a small ball about a centimetre in diameter, sus- 
pended by a fine thread, is placed in the position 
previously occupied by the perforation in the screen, 
it will cut off those rays which, by themselves, would 
form a positive inverted image on the screen. The 
part of the screen which was previously rendered 
bright by the rays which passed through the perfora- 
tion in the card is now rendered dark by the inter- 
ception of these rays, and the shadow of the ball 
takes the form of a dark crescent, which is'a negative 
inverted image of the bright crescent from which the 
rays emanate. The ball must be placed so far from 
the screen that the umbra of the shadow is not 
formed; for the rest, there is considerable latitude as 
to the position in which the ball is placed. A body 
of about the same size, but with a shape differing 
considerably from the ball, casts an identical shadow. 
If a larger body is used, its shadow still has the 
form of an inverted negative image of the extended 
source of light, but the definition is less perfect. 
If the disc which partly covers the illuminated tissue 
paper be removed, and a sheet of card, in which a 
stencil letter has been cut, is substituted for it, an 
inverted negative image of the stencil letter is 
obtained on the screen. Instead of the stencil letter, 
we may use a letter painted in transparent red on the 
tissue paper, the remainder of the illuminated area 
being painted blue; in this case the shadow of the ball 
thrown on the screen takes the form of an inverted 
image of the letter in blue, on a reddish ground. 
These experiments show that when light from am 
