78 
glowing-point ; again a dark interval follows, shorter than the 
first, and behind it a long strip with a dark core and very faintly 
bright edges ; as one traces backwards, the edges appear to 
close in together gradually, so that, after about two inches, the 
dark core has collapsed, as it were, and the edges have come 
together to form a narrow and well-defined thread of a mauve 
tinge ; this gradually dies away as we go further back along the 
trail, and by the time that the glowing-point has travelled over 
the whole curve once, it has nearly disappeared. 
Secondly, let the figure of eight be as large as can be de- 
scribed in a rectangle 8 inches by 4. Here the phenomena are 
quite different. It now seems as if the dark intervals at either 
end of the ghost as described above were absent, and the ghost 
itself were drawn out into a streak which follows ¢mmedia/ely upon 
the glowing-point. Its colour is now yellow-green. This 
gardually narrows to extinction as one traces the trail backwards, 
and is the positive after-image in its various stages. More pro- 
bably this streak has no connection with the true ghost; but is 
quite distinct from it, whilst the ghost no longer appears, when 
the point moves with greater velocity. In fact, there is probably 
a limiting velocity of the glowing-point, beyond which the ghost 
is not formed. This coincides with Mr. Bidwell’s observations 
as to the rate of rotation of the vacuum-tube. As the yellow 
streak disappears narrowing, one sees a faint blue haze on either 
side, separated from it by an interval of darkness. When one 
has traced backwards so far that the streak has vanished, one 
sees what was above described as a strip with dark core and 
faint blue or mauve edges. The edges close in and form a 
distinct mauve thread, which gradually dies out. 
It is very beautiful to see the ghostly trail hanging before 
one ; and, by suitable movement of the glowing point, one may 
fill the space, as it were, with a maze of wreathing lines. Per- 
haps the most s'riking part of the phenomenon, regarded from 
an esthetical standpoint, is the def¢’ of the figures so produced : 
one realises in the form of the trail that the glowing-point has 
been moving, not in one plane, but in space ; and one sees that 
some parts are nearer than others. After a time the glowing- 
point seems to be forgotten, and the trail is the only thing ob- 
served. The position of the trail appears to change with any 
change in the state of accommodation of the eye; if the trail 
goes away from one the eye attempts to follow it, and exag- 
gerates the movement. If there is any irregularity in the curve, 
as may often be the case from want of proper co-ordination of 
muscles—especially if the moving arm is at all subject to 
rheumatism—it is revealed in a terribly truthful manner by the 
trail. 
A systematic investigation of the subject would, I think, be 
very valuable as throwing light upon the processes in the retina. 
Both Prof. Young (‘‘ whatever the true explanation may turn 
out to be, the phenomenon at least suggests the idea of a 7e/lec- 
tion of the nervous impulse at the nerve extremities, as if the 
intense impression upon the retina, after being the first time 
propagated to the brain, were then reflected, returned to the 
retina, and, travelling again from the retina to the brain, 
renewed the sensation”) and Mr. Bidwell (‘‘the series of 
phenomena seem to be due to an affection of the optic nerve 
which is of an oscillatory character,” &c ) appear to incline to 
what I may call a physical view of the phenomena, The 
phenomena appear to me to point to some chemical action 
on the retina, and to depend in a great measure on the ra/e 
at which this action goes on. It would be of great interest to 
consider the phenomena in connection with Hering’s theory of 
colour sensation ; according to it these sensations are due to 
changes in a certain substance, in such a way that changes of a 
destructive or dissimilative character give rise to the sensations 
of white, red, and yellow, whilst those of a constructive or 
assimilative kind produce the sensations of black, green, and 
blue (‘‘Zur Lehre vom Lichtsinne,” Wien, 1878). It may be 
that this work has been already done ; if so I must crave the 
indulgence of those who have made the subject a special study, 
H. Frank NEWALL 
Crowthorne, Wokingham, May 18 
“Speed” and ‘‘ Velocity” 
Some of your ‘‘general” readers, like myself, may wish to 
see the distinction between ‘‘ velocity” and ‘‘speed” more easily 
defined than by a reference to the calculus of quaternions, to 
which I believe the term ‘‘ tensor” appertains. 
‘« Speed” is not in the index to the new edition of Part II. of 
NATURE 
a  ———  — 
[May 28, 1885 
Thomson and Tait. Maxwell, at p. 26 of ‘‘ Matter and Motion,” — 
says, ‘‘ The rate or speed of the motion is called the velocity of — 
the particle.” Tate, in his ‘‘ Properties of Matter,” p. 52, | 
writes about ‘‘ water of motion; 7.e. Speeds.” It seems thus:— 
(1) Rate of motion is velocity (Maxwell) 
(2) Speed of motion is velocity Prien 
(3) Rate of motion is speed (Tate). ~ 
From (1) and (3) it appears as if velocity and speed must be the 
same, as indeed (2) seems to assert. But we are told this is not 
the case. Cannot the distinction between the two be made 
more generally intelligible than by saying that ‘‘speed”’ is the 
“tensor” of velocity. SENEX 
[When Maxwell introduced to junior students the Diagram 
of Velocities, he made velocity include the direction of motion as 
well as the mere va/e of motion (z,e. speed). —ED. ] 
The Male Sole is not Unknown 
In la-t week’s issue of NATURE is what is said to be an 
abstract of a paper read at the Society of Arts by Prof. Ray 
Lankester, in aid of a proposed marine laboratory, and, passing 
over what he stated generally requires elucidation, he gives one 
example of what zs not known among fishes, and which in the 
first instance will be investigated at Plymouth. He is made to 
say ‘fat present absolutely nothing is known as to the spawning 
of the sole—the male fish is not even recognised.” 
In times gone by the plaice was asserted to have ascended 
from a shrimp, but this, I think, is the first time that the exist- 
ence of the male sole has been declined recognition. Omitting 
references to others, I will merely draw attention to the fact that 
in my collection of British fishes in spirit at the ‘* Great Inter- 
national Fisheries Exhibition,” and which is now deposited in 
the Economic Museum at South Kensington, is a fine example 
of the male sole, with the milt quite ripe. 
YT must apologise for pointing out the foregoing, but were such 
an error left unnoticed in a scientific paper, some practical 
fisherman will possibly direct attention to it, as the comparative 
rarity of the male to the female sole has been frequently observed 
upon in our weekly sporting journals during the Jast few years. 
Cheltenham, May 23 Francis DAy 
The Aurora of March 15, 1885 
Nature for March 26 (p. 479) contains an account of a fine 
aurora observed at Christiania, Sweden, on March 15, by Prof. 
Sojhus Tromholt. I would call attention to the fact that an 
aurora (a very unusual phenomenon at this place) was visible 
here on the evening of March 15. [t was first seen at about 
7 p.m, 
At the above time s veral streamers were noticed ascending 
somewhat east of north: after a short interval these died leaving 
a white nebulous cloud of light at an altitude of about 10° near 
a point some 10° or 1§° east of north. Shortly afterwards 
streamers appeared ascending some 10° or 15° west of north ; 
these presently disappeared, leaving a mass of light similar to 
that left in the east of north. Several times feebler streamers 
made their appearance west of north. The rays did not attain a 
greater height than sone 20°, and by 8}h. all was quiet, save an 
auroral glow along the horizon some few degrees east of north, 
which remained throughout the night. I have thought this 
might be interesting in connection with the Christiania aurora. 
oh. 39m. 0°68s. 
=+36h. 8m. 58 25s. 
E, E. BARNARD 
Vanderbilt University Observatory, Nashville, Tenn., U.S.A. 
Longitude west of Washington = 
Latitude 
Catalogue of Fossil Mammalia in the British Museum. . 
Part I, | 
In reply to Mr. T.ydekker’s comments on the review of his 
work (NATURE, vol. xxxi. p. 597) I an glad to find that the 
author repudiates the Owenian system and its errors, though his — 
recognition of the three upper premolars in Vesperé#lio as corre- 
sponding, respectively, to pms. 2, 3, and 4 of the typical series of 
four, and the minute anterior upper premolar of Riinolophus as 
p. 3, added to the strange absence of any note on the presence ~ 
of exceptions to the supposed rule that the premolars decrease | 
in number by reduction from the anterior extremity of the series 
