Aveust 13, 1897. ] 
Dr. R. S. Curris, of the University of Chi- 
cago, has been elected professor of chemistry 
in Hobart College. 
Mr. Jonn P. Hytan has been appointed in- 
structor in experimental psychology in the Uni- 
versity of Illinois. 
Dr. Hope, lecturer on hygiene at University 
College, Liverpool, has been made professor. 
Dr. TRAUBE, Privatdocent at Berlin, has 
been appointed to the newly established pro- 
fessorship in the Technological Institute at 
Charlottenburg, and Dr. Adalbert Kolb, Privat- 
docent in chemistry in the Technological Insti- 
tute at Darmstadt, has been promoted to a 
professorship. 
DISCUSSION AND CORRESPONDENCE. 
CEREBRAL LIGHT. 
In SciENcE for July 23d, p. 138, I find a letter 
from Dr. Scripture in which he makes some 
very acute observations on the the origin of the 
figures, usually irregular and obscure, but some- 
times quite definite, which are seen in the dark 
field of the closed eyes. In past years I have 
spent many hours in studying these figures and 
they are briefly described in my little volume 
on Sight, pp. 66 and 67 of last edition. They 
are usually considered as of retinal origin and 
sometimes spoken of as ‘retinal light ;’ but Dr. 
Scripture gives what he thinks conclusive rea- 
sons for thinking that they are of cerebral origin, 
and therefore proposes the name ‘cerebral light.’ 
Now, as to the question of origin, I am not pre- 
pared to say anything. I wish now only to 
show that his supposed tests are not valid. 
1. He says that with the eyes closed there is 
but one dark field, instead of two, as there 
ought to be if its origin is retinal ; for there are 
two retinze. Now, if he means the simple field 
without reference to the figures in it, I would 
ask: How could there be more than one? Even 
with the eyes open, there seems to be but one 
field. Only by close observation can we see 
that there are really two partly overlapping 
fields forming a common field bounded on the 
two sides by the faint images of the nose. But 
in the dark field there are no images of the 
nose. Butif, on the other hand, he means that 
SCIENCE. 
257 
the two retine could not be expected to be 
similarly affected in all parts, and therefore 
there ought to be different figures for the two 
retinge in the same dark field, then I would 
ask again: How are we to distinguish the fig- 
ures belonging to each retina in the one dark 
field ? 
2. But, in further proof, he says: These 
figures do not move with the movements of the 
eye ; while after-images, which are admittedly 
retinal affections, do thus move. Now, I find, 
on the contrary, that these figures behave ex- 
actly as the after-images do. I find that, in look- 
ing in a different direction in the dark field, 
they may indeed disappear, but only to reap- 
pear at the new point of sight. After-images 
do the same. Unless they are very strong, 
they also, on changing the point of sight, dis- 
appear to reappear at the new point. 
It is possible, however, that we are talking 
about different things. It is possible that there 
are two different kinds of figures in the dark 
field, one retinal and the other cerebral. 
3. But, again, he says that these figures 
do not change place when the axis of the eye 
is displaced by pressure in the corner, whereas 
after-images do change place under these con- 
ditions. Now, on the contrary, I find that 
after-images under these conditions do not 
change place. It is true that with the eyes 
open they may seem to move, but this is only 
an illusion, the result of the contrary motion 
of all objects in the field of view. Real ob- 
jects move because their images change their 
places on the retina while we look in the same 
direction, but the retinal brands which cause 
after-images cannot change their places on the 
retina. But now shut the eyes, so that there 
are no objects to plague us; then we find that 
after-images do not move by displacement of 
the axis of the eyes. There is only one case 
(that of the previous head) in which after- 
images follow the motions of the eye, although 
it is the commonest case. It is that in which 
the two eyes move together in the same direc- 
tion. In other words, they follow the direction 
of looking, not the direction of the individual 
eye. But in displacement of the eye-axis by 
pressure we do not change the direction of the 
looking of the observer. 
