Frerruary 18, 1904] 
NAT ORE 
on9 
that the fovea centralis is the special point of vision when 
looking at an object and fixing our eyes upon it. 
(foveal), the rods are excluded, and it is only in indirect 
vision (peripheral) that they come into action. Thus then 
in dim light these two elements enter into a sharp contest 
which, if the light is dim enough, results in favour of the 
colour-blind rods, so that everything then resolves itself 
into greys, i.e. colourless shades of light. 
By the help of this theory one gets a natural explanation 
of phenomena hitherto unexplained, as, for example, 
Purkinje’s phenomenon, the change of position of the 
““neutral point ’’ in the spectrum as light decreases in the 
case of those who confound red with green, and the depen- 
dence of colour identification on the absolute intensity of 
light. In my work ‘* Grauglut und Rotglut ’’ I was able 
to show? that the remarkable ** shadow-like ’’ (gespenster- 
haft) appearance of the grey and red glow can be ex- 
plained by attributing to the two light-perceiving elements 
the part assigned to them by v. Kries. 
If in a dark room we observe the gradual rise in tempera- 
ture of a body from that of the room up to glowing tempera- 
ture, then, according to my view, the eye perceives two 
sudden changes or “‘leaps,’’ first from dark to shadowy 
grey (“‘ grey glow ’’), and later from grey glow to coloured 
glow (red glow). 
stepping over the threshold of stimulation of the optic nerve, 
but the efficient organs are not the same in the two cases; 
the grey glow corresponds to the threshold of stimulation 
of the rods, the red glow to the threshold of stimulation of 
the cones. Accordingly we must conceive of the grey glow 
as a sensation of the retinal rods and of the red glow as 
the sensation of the retinal cones. 
The “ shadow-like ’’ character of the rod-vision is not 
apparent until we observe a sufficiently small surface the 
retinal image of which does not exceed in area that of the 
spot of clearest vision, i.e. the fovea centralis, and the in- 
crease of brightness of which we follow in the dark from 
zero upwards. For this purpose it is best to make use of 
a platinum plate brought to a glow by means of electricity 
and limited by a diaphragm, the development of light thus 
being observed in the dark by a well-rested eye. When the 
platinum plate has reached a temperature of about 400° C., 
at first only the rods of the eye searching in the dark are 
stimulated, and the perception of colourless light (grey 
glow) is aroused in the brain. 
Being accustomed to gaze at what sends us light we turn 
our eyes im the direction from which we believe the light 
rays come. As, however, the’ cones have not yet been 
stimulated, the fovea centralis sends no message of light 
to the brain; accordingly we cannot see the spot gazed at. 
Thereby we are confronted by the remarkable fact that we 
see something which we are not gazing at, whilst it be- 
comes invisible when we wish to fix our eyes upon it. And 
as we can see nothing by direct vision, we involuntarily 
move our eyes away, whereby the rays again fall on extra- 
foveal retinal spots; we again receive the impression of 
light, and our search after the place from which the re- 
markable light comes begins over again. Thus there arises 
in us the impression of a light which darts to and fro, which 
is sometimes present, then again evades us, mocking us, 
like a will-o’-the-wisp. It is only when the brightness is 
of a sufficient intensity to stimulate the cones also and 
enable them to send a message of light to the brain that 
this unusual condition comes to an end, and then we see 
what we gaze at, just as we have been accustomed to do, 
ani the thing seen no longer escapes the searching gaze. 
In the case of glow this does not occur until the body 
has reached a temperature somewhat above 500° C.; not 
until then are the cones stimulated, and we then perceive 
colour as well as brightness, in other words, the ‘‘ grey 
glow ”’ is transformed into ‘‘ red glow.”’ 
But at a still higher temperature (up to 700° C. and 
above) the rods enter into vigorous competition with the 
cones, and the light red colour seen in gazing at the 
platinum plate changes in indirect vision into a peculiar 
colourless white, the ‘‘ rod white,’’? while at the same time 
the brightness of the platinum plate increases considerably. 
1 O, Lummer, ‘“‘ Uber Grauglut und Rotglut” (Wied. Anv., Ixii., 14-20, 
1897 ;; Verh. Phys. Ges. Berlin, xvi., 121-127, 1807). 
NO. 1790, VOL. 69] 
In each case the ‘‘leap’”’ arises from | : a s : B le 
£ | outlines on darkening it while the gaze is fixed on it, in 
Hence | 
it follows that in gazing at an object, i.e. direct vision | * shadowy vision 
Ia some of Blondlot’s experiments one finds oneself in 
precisely the same position as in the observation of the 
” just described. One perceives a very 
small slightly luminous surface, e.g. a dull red glowing 
| platinum plate, in the dark and fixes one’s gaze upon it. 
Before bringing one’s undivided attention to bear on it, it 
is seen by the extra-foveal parts of the retina, because the 
eye involuntarily endeavours to gather as much light as 
possible, thus consequently both rods and cones take part 
in the vision. As soon, however, as the lead screen or the 
hand is interposed between the source of illumination and 
the luminous platinum surface, the observer, in order to see 
the change in it, will fix his gaze as directly as possible on 
the platinum plate, thereby excluding the rods. The 
necessary consequence will be that the platinum plate will 
appear reddish and less bright, and the rod-white of the 
peripheral parts of the retina be lost. But this fixing 
of the gaze requires time and effort. The darkening and 
the red colouring observed will also require a certain amount 
of time, and as soon as the hand or the screen is removed 
the eye will return as quickly as possible to extra-foveal 
observation, in which it receives more light. After the re- 
moval of the screen, therefore, the brightness of the 
platinum plate increases, and provided the brightness of 
the luminous surface under observation is very dim, there 
will be an immediate diminution in the distinctness of the 
fact, there will eventually be a complete disappearance of 
the platinum plate provided the energy sinks below the 
threshold of stimulation of the cones and the surface is 
small enough. 
As a proof that the phenomena here described (which were 
not only observed by myself subjectively, but were produced 
in my lecture before a large audience) resemble to an extra- 
ordinary degree the more recent observations described by 
Blondlot, I will quote, word for word, two sentences from 
his article of November 2, 1903 (C. R., cxxxvii., 655, 1903). 
After describing the order of procedure in the observation 
of a feebly illumined strip of paper, he goes on to say :— 
| ‘“ If one now intercepts the rays by interposing a lead plate 
or the hand, one sees the small rectangle of paper grow 
dark to its contour and lose its distinctness; the removal of 
the screen causes the brightness and the distinctness to 
reappear, the light diffused by the strip of paper being then 
increased by the action of the n-rays.”’* 
In the case in which Blondlot the 
observes trans- 
| parently luminous paper mirrored on a needle and then 
illumines the needle with the n-rays, he describes the process 
in the following words :—‘‘ It was then easy to prove that 
the action of these rays strengthens the image, for if one 
succeeds in intercepting them, this image becomes dark 
and reddish. I have repeated this experiment with equal 
success by employing instead of the Initting-needle a plane 
bronze mirror.’’* 
In this article it is also stated :—‘‘ All these actions of 
the n-rays on light require an appreciable time for their 
production and disappearance ’’;* this is on a parallel with 
the appearances of grey and red glow during vision in the 
dark. Lhe experiments described in this article do not deal 
with the behaviour of different substances under exposure 
to the n-rays. It is unnecessary to say that seeing in the 
dark can in no wise explain why some substances transmit 
the n-rays and others do not. But it may be asserted 
bricfly that neither brightening, darkening, nor change in 
colour will take place if during the experiment with the 
above mentioned source of light one gazes continuously at 
the analysing luminous surface so that the image always 
| falls on the fovea centralis and the cones alone come into 
action. As a matter of fact, Prof. Rubens, as he kindly 
informed me in answer to my question, took his obsery- 
ations in this way, and could perceive no brightening even 
i “Si maintenant on intercepte les rayons en interposant une lame de 
plomb ou la main, on voit le petit rectangle de papier s’assombrir, et ses 
contours perdre leur netteté ; |'éloignement de l’écran fait reparaitre l’éclat 
et la netteté ; la lumiére diffusée par la bande de papier est donc accrue par 
l'action des rayons #.” 
2 “11 fut alors facile de constater que l’action de ces rayons renforce 
l'image, car si l’on vient A les intercepter, cette image s'assombrit et devient 
rougeatre. J'ai répété cette expérience avec le méme succes en employant, 
au lieu de l’aiguille 4 tricoter, un miroir plan en bronze.” 
% ‘Toutes ces actions des rayons » sur la lumiére exigent un temps 
appréciable pour se produire et pour disparaitre.” 
