May 14, 1885] 
NAT ORE 
31 
follow one another at the rate of 7 per second, the num 
ber of images simultaneously visible will be about 7/8) 
since the luminous image produced by each separate 
flash persists for about an eighth of a second after the 
flash itself has ceased. The result of these effects is the 
appearance of a gorgeous revolving star. 
If the tube is made to rotate very slowly, there occurs 
a different and very curious phenomenon, which, so far 
as I know, has never hitherto been noticed. The tube 
used in my experiments was thirteen inches long, and 
contained various devices in uranium glass ; the induc- 
tion-coil had a resistance of 1400 ohms, and was worked 
by’a single large bichromate cell. When the rotation is 
performed at about the rate of one turn in three seconds, 
the luminous images of the tube are almost superposed, 
forming a bunch which is slightly spread out at the ends. 
But about 40° behind the bunch, and separated from it 
by an interval of darkness, comesa ghos¢. This ghost is 
in shape an exact reproduction of the tube: it is very 
clearly defined, and distinctly shows every detail of the 
uranium glass devices. But the colour is entirely changed, 
the violet tint of the luminous bulb and the bright green 
fluorescence of the uranium glass being replaced by a 
uniform steel gray. If the rotation is stopped, the ghost 
still moves slowly on, and, after the lapse of about half a 
second, disappears in coalescing with the luminous tube. 
The phenomenon may be diagrammatically represented 
by the letter X, the thick stroke being the bunch of 
luminous images, and the thin stroke the spectral at- 
tendant. The direction of the motion is supposed to be 
opposite to that of the hands of a watch when seen from 
above. If the rate of rotation is too slow, the ghost ap- 
proaches the luminous bunch so closely as to be obscured 
by its superior brilliancy; while, if it is too fast, the 
image becomes blurred and ill-defined. | The strength of 
the inducing current should be regulated by trial. With 
too strong a current the effect is the same as when the 
rotation is too slow; with too weak a current the image 
is rendered feeble. Generally speaking, the best results 
are obtained with a somewhat weak current. 
The experiment has been witnessed by a dozen persons, 
all of whom, with the exception of one adult, and the 
doubtful exception of a child, at once saw the spectral 
image. It is almost ludicrously difficult for those who 
are able to see it, to understand how any one else could 
possibly fail to do so. 
This curious effect clearly belongs to the class of 
spectral images or “ocular spectra,” which result from 
looking at a bright object, persistence of vision in the 
ordinary sense of the term having nothing to do with it. 
I proved this to be the case in a very simple manner. The 
vacuum tube being at rest ina feebly-lighted room, I con- 
centrated my gaze upon a certain small portion of it 
while the discharge was passing. The current was then 
interrupted, and the luminous image was almost instantly 
replaced by a corresponding image which appeared to 
be intensely black upon a less dark background. After 
a period which I estimated at from a quarter to half a 
second (probably more nearly the latter), the black image 
again became luminous, assuming the characteristic steel 
gray colour: this luminous impression lasted but for a 
small fraction of a second, and the series of phenomena 
terminated with its disappearance. I found the effect to 
be most clearly marked when a narrow portion of the tube 
was observed ; the definition of the spectral image was 
then exceedingly sharp, even the striz being represented 
with perfect distinctness. It was also found desirable to 
make the preliminary illumination as short as possible, a 
single flash being generally sufficient to produce the 
phenomena. This is more easily effected by a judicious 
manipulation of the contact-breaker than by means of a 
key, or of the commutator attached to the coil. I may 
add that it is by no means certain that a person who is al- 
together new to the subiect will at first be able to 
~ee the appearances last described, even when he knows 
exactly what to expect. They belong to a class of 
phenomena which in ordinary life we habitually train 
ourselves to disregard, and our persistent neglect makes 
it difficult to perceive them when we desire to do so. 
With a little patient attention the difficulty will probably 
disappear. It was probably owing to my constant habit 
of studying visual impressions that the appearance of the 
ghost attracted my notice in the first instance. 
The series of phenomena seem to be due to an affec- 
tion of the optic nerve which is of an oscillatory charac- 
ter. Abnormal darkness follows as a reaction after the 
luminosity, and again after abnormal darkness there is 
a rebound into a feebler luminosity. Following this idea 
I have endeavoured to detect the existence of a second 
ghost as the result of a further rebound, but hitherto 
without success. 
It is an interesting fact, as proved by these experi- 
ments, that the formation of a spectral image does not 
occur until the expiration of a measurable interval of 
time after the exciting cause has ceased to operate. 
SHELFORD BIDWELL 
¥UPITER 
URING the present opposition of this planet, the 
details of the belts and spots have continued to 
furnish materials of great interest. Some very obvious 
modifications have occurred since the previous year, and 
several curious new features have become conspicuous. 
The great red spot has surprised us by its extended dura- 
tion. As early as 1882 it lost such a considerable depth 
of tone that its obliteration seemed imminent, but it has 
lingered on, until now its existence appears likely to be 
indefinitely prolonged, though under visible conditions 
far less imposing than at an earlier stage. All that at 
present remains of this remarkable formation is a dusky 
elliptic ring, darkest at the following end, and only well 
seen under good definition. Whether this ellipse is 
identical with similar appearances delineated by Dawes 
in 1857, Huggins in 1858, and Gledhill in 1869, 1870, and 
1871 is involved in doubt, because of the lack of inter- 
mediate observations. We have no definite information 
as to what became of the various objects alluded to. It 
is very possible that they severally represent an object of 
considerable permanency. ‘The changes such as observed 
may have been induced by atmospheric interference. 
There is every indication that the dense vaporous en- 
velopes of this planet are rapidly variable, especially in 
the zone included by the two equatorial belts, and that 
the chief features undergo singular fluctuations, some of 
which may possibly be of periodical character. 
The particular objects drawn by Dawes and others 
suggest a close relationship to the red spot as it now 
appears. There is far from being an actual coincidence 
either in the positions or forms of the features here sought 
to be connected, but small differences must actually occur 
in results based on estimation. A sufficient likeness is 
established between them to show that further investiga- 
tion may have an interesting outcome as affirming the 
theory of recurrent markings of identicalform. There is, 
however, an inability to trace the history of these singular 
objects, owing to the meagre number of observations 
available. This is a circumstance much to be regretted. 
Markings of specially interesting character deserve some- 
thing more than mere record. They should be persistently 
watched during several oppositions, if possible, for it is 
only by this continuity of records that the really important 
questions affecting them admit of settlement. The red 
I The adult who failed to see the ghost is totally unable to perceive the 
subjective images in complementary colours, which generally result from 
gazing at brightly-coloured objects. Her general powers of vision are 
decidedly above the average. and she is in no degree colour-blind. The 
doubtful child is a daughter of this adult. A younger child can certainly 
see the phenomenon. 
