(2) The brightness of the superposed spectrum increases proportionally to the breadth of the slit.

(3) With an oscillating or rotating slit the brightness of the superposed spectrum remains unaltered; that of the image of the protuberance decreases according to a law which depends upon the number and duration of the impressions produced on the place of the retina in question in the unit of time, and on the refrangibility of the observed protuberance-band.

Assuming, for simplicity's sake, that the entire surface overwhich the slit moved in its rotation or oscillation were occupied by the protuberance, and assuming that the intensity of the after-image formed were inversely proportional to that surface (corresponding to a uniform distribution over that surface of the light passing through the stationary slit), then assuming the above three principles, the ratio of the intensity between ground and protuberance would remain the same, whether,

First, by oscillation of the slit the brightness of the image of the protuberance were diminished, and thus the brightness of the superposed spectrum or of the ground (according to (2)) were left unchanged, or whether,

Secondly, the stationary slit was so far opened that its aperture just extended over the space over which in the first case the oscillation extended. Hereby, according to (1), the apparent brightness of the protuberance would remain unchanged, while that of the ground would be increased in the same ratio in which it was formerly weakened when the ground was unaltered.

Hence, under the above suppositions, the intended object would be far more simply attained in the second way, by taking care that, on account of dazzling, the intense direct light of the sun did not penetrate into the slit.

The slit need then only be opened so far that the protuberance, or a part of it, appears in the aperture. By polarizing or absorbing media, placed in front of the eyepiece, a suitable weakening of the entire field of view must be provided for, in order that the ratio between the intensities of the protuberance and superposed spectrum may be as striking as possible.

Led by these considerations, I have attempted to realize by means of terrestrial sources of light the conditions under which the protuberances are visible, in order thus to test both methods and convince myself of their practicability. In order the better to understand the experiments described, the following remarks may be premised.

The reason why, under ordinary circumstances, by deadening the intense solar image the protuberances are not visible at its edge, lies in the superposed strongly illuminated particles of our atmosphere. In a total solar eclipse this superposed light is so

considerably weakened, that then the intensely luminous protuberances stand out from the illuminated parts of the corona of the darkened sun. We may form an idea of the magnitude of the necessary enfeeblement of the diffuse light of our atmosphere, if we assume that the mean luminosity of the atmosphere during a total solar eclipse is equal to that during an average full moon. From my photometrical measurements* this luminosity is 618,000 times less than that produced by the sun. Hence the selective absorption of coloured media must stand in a similar ratio to that of the homogeneous light of the protuberance, if, as is attempted on various sides, we wished to make the protuberances visible without dispersion.

On the other hand, the possibility of attaining this object by the aid of the prism by dispersing the superposed atmospheric light depends essentially upon the circumstance that this light consists of rays of all refrangibilities, while that of the protuberances only consists of three homogeneous kinds of rays.

I have in the following manner produced artificially the superposition of a non-homogeneous mass of light over a body shining with homogeneous light and bounded by sharp outlines.

The wick of an alcohol-flame was impregnated with chloride of sodium and chloride of lithium. At a distance of eighteen feet from this flame, a piece of plate glass was so placed at an angle of 45° to the direction of observation, that the reflected image of a petroleum-flame at the side covered the feebly luminous alcoholflame, and by its considerably greater intensity rendered it quite invisible. About a foot in front of the reflecting glass plate was a small lens of 6 inches focus, which threw an image of the alcohol-flame upon the slit of the spectroscope. The latter was fastened to the end of a spring about 10 inches long, by which, removed from its position of equilibrium and left to itself, it could for about five minutes be made to perform oscillations of sufficient amplitude.

The breadth of the slit was first of all so far diminished, that when the slit was at rest the double line D, and in comparison feebly the lithium-line, appeared well defined in the field.

When the slit was made to oscillate, these lines changed into sharp images of the alcohol-flame, of which the two soda images were about half covered. The apparent brightness of these three images was considerably smaller than that of the bright lines, and hence their prominence on the diffusely illuminated spectrum-ground smaller in the same ratio than that of the lines when the slit was at rest.

When now I applied the second of the above proposed methods, and opened the stationary slit so far that the image of * Photometrische Untersuchungen &c. p. 105. Leipzig, 1865.

the alcohol-flame was just bounded by the rectangular slit, I was surprised by the far greater beauty and distinctness with which the images of the flame stood out from the diffusely illuminated spectrum-ground.

I may remark that I used in this experiment only one of the above-mentioned newer prisms; but it is clear that, with increasing dispersion, the enfeeblement of the superposed non-homogeneous light may be enhanced at pleasure.

In principle no difficulties prevent the application of this method to the sun's protuberances *. Yet practical success, with the given ratio of the intensities of homogeneous protuberance- and superposed atmospheric light, is essentially dependent on whether a sufficiently strong dispersion for this ratio can be attained. If, however, from the intensity and distinctness with which the lines of the protuberances appear, especially the middle one (of which I have convinced myself by my own observation at the Berlin Observatory on the 24th of last December), it is allowable to infer a very considerable relative brightness of the protuberances, the means now at my disposal (four excellent systems of prisms) will probably be sufficient to solve satisfactorily, in the way here proposed, the problem of the visibility of protuberances.

Leipzig, February 1869.

Appendix.

M. Faye, after giving an account to the Academy of Sciences, on September 20, of the above paper, proceeds as follows:

"M. Zöllner has subsequently applied his new method to the sun with the most complete success. He has been able to follow and map from minute to minute with surprising facility and accuracy the magnificent phenomena of the chromosphere; he is even about to photograph them, utilizing the images due to the ray situated in the most photogenic part of the spectrum.

"Some of the drawings above mentioned have been published by Zöllner in a separate pamphlet. They show clearly that the protuberances are violent eruptions (Mr. Lockyer has already approximately determined their velocity), and not clouds suspended in an atmosphere. They might be said to consist of a gaseous mass projected vertically into an almost vacuous space, expanding almost immediately, and then falling more slowly, assuming the most capricious forms. Perhaps in this way we shall be able to group the new manifestations of the force which the sun exerts upon the very light material of comets,—a polar force, according to Bessel and Olbers, like electricity and magnetism; a force merely repulsive according to another hypothesis,

*Owing to my not having yet completely set up the necessary instruments, I have been unable actually to test this method.

In

with which M. Roche's beautiful researches are connected. any case these drawings, which refer to four days, give the key to a very curious enigma presented by the eclipses observed in South America, in Chili, and in Brazil; I speak of the black protuberances. They seem to me to be due merely to the dark interval which exists for a few minutes either between two adjacent eruptions the plumes of which join, or between the ascending column of an eruption and its plume falling on the side of it.

"Thus to observe the protuberances with the spectroscope at any hour of the day, even when the sun is near the horizon, it is sufficient to open slightly the slit of the spectroscope. Perhaps M. Zöllner will succeed in seeing them all together as in an eclipse, by using very large prisms and a slit curved as an arc of a circle."

XLIII. On the Structure of the Human Ear, and on the Mode in which it administers to the Perception of Sound. By R. MOON, M.A., Honorary Fellow of Queen's College, Cambridge.

Ν

[Continued from p. 130.]

IN my last paper I endeavoured to show :—

:

1. That the fact of the tympanal membrane being concave outwards, coupled with its flexibility, adapts it as an agent for the transmission to the sensorium of the motion arising from rarefied waves, while the same concavity, coupled with the inelastic and unyielding character of the membrane, forbids the transmission of the motion arising from condensed waves.

2. That if the ear yields to the impressions which rarefied waves tend to produce upon it, an apparatus will be required by means of which, after exposure to such waves, the membrana tympani may be brought back to its original position, and the organ generally be restored to its normal status; that the muscles acting upon the bones of the ear are calculated to perform that office; and that no other adequate function has ever been assigned to them; whence we may conclude that that portion of the auditory apparatus has been contrived with exclusive reference to the action upon the ear of rarefied waves.

3. That when either the tympanal membrane or the malleus or incus is wanting, or the latter of those bones is disconnected from the other or from the stapes, then, under the influence of rarefied waves, the oscillations between the vestibular and cochlear fenestræ of the fluid in the labyrinth will still be maintained by the alternate action, on the one hand of a difference in the external pressures upon the fenestræ, and on the other of the stapedius muscle; and that in this way a considerable power of

perception of sound may occur; at the same time, that when the ear is exposed to the action of condensed waves under the same circumstances no motion of the fluid in the labyrinth, and cousequently no perception of sound can occur.

The question here naturally presents itself, if, when the membrana tympani, malleus, and incus are wanting, and the Eustachian tube ceases to perform any recognizable function, hearing occurs in a manner, in some cases, not very much less perfect than when the ear is in its normal state, how comes it that a machine so much more extensive and complicated is ordinarily resorted to by nature for the accomplishment of that object?

To this it has been replied, that in the perfect ear the machinery is much more efficiently protected from external injury, whether arising from foreign bodies which find their way into the meatus, or from cold*, than is the case with the mutilated

organ.

It may be added, moreover, that, on the view of the auditory apparatus above set forth, the unyielding character of the tympanal membrane must operate to protect the organ from injury arising from condensations of the atmosphere, while the opposite actions of the tensor muscle and of rarefactions of air must tend to mitigate the effect upon the organ of the latter.

It may readily be conceived, too, in a general way, that the ear in its normal state must be a more powerful, more refined, and more manageable instrument than that presented by the simple labyrinth with its contents and closures, aided by the stapedius muscle only.

A more important consideration, however, still remains.

If we regard the importance and delicacy of the functions performed in the perfect ear by the two muscles combined and in the imperfect ear by the stapedius alone, if we consider that these muscles are under the influence of nerves which are not involuntary but are subject to the action of the will, if we advert to the very slow and gradual manner in which the recognition of articulate sounds is developed in infancy, if we take account of the apparently boundless interval between the capacity for appreciating sounds possessed by the obtuse rustic and by the finest musical genius-if we keep in view these various facts, I think it cannot but be evident that a long and most delicate process of education of the nerves and muscles must be passed through before that degree of proficiency is attained which is requisite for the comprehension of spoken language, and that one still more extended and refined must be undergone

*The inconvenience from this latter cause, when the membrana tympani is absent, is often very great. See papers by Sir Astley Cooper in the Philosophical Transactions for 1800 and 1801.