individual by constant practice from the earliest childhood, that a single act of volition accomplishes them all. Under ordinary circumstances they are so indissolubly associated that neither can be accomplished without the others. But the experiments described above prove that under certain circumstances the first two at least may be completely dissociated. In these experiments, when the image is first obtained, the optic axes, the lenses, and the pupil are all consensually adjusted for vision at the distance of the image; and hence the image must be indistinct, for the rays diverge from an entirely different distance. But gradually the lenses adjust themselves to the actual divergence, i. e. for rays diverging from the real object, while the optic axes remain adjusted for the distance of the image. The difficulty experienced in dissociating these two adjustments causes the interval of indistinctness. The perception of the difference between the image and a real object is the sense of this dissociation. Consensual movements have been perhaps brought about by the necessities of single and distinct vision; Helmholtz has shown* that other consensual movements may be dissociated when the necessities of single vision require it; these experiments show that the consensual adjustments of the eye may be dissociated when the necessities of distinct vision require it.

I was now anxious to determine what part was taken by the pupil. Is the contraction of the pupil more intimately associated with the axial or the focal adjustment? This question has been discussed by E. H. Weber, Cramer, and Donderst. Weber believes it is directly associated with the axial adjustment, Cramer and Donders with the focal adjustment. To test this question, while I was obtaining the image and making it clear, an assistant standing behind and a little to one side observed my pupil reflected in a small mirror conveniently placed. After gazing intently at the real object until the pupil was steady, as soon as I converged the optic axes so as to obtain the image No. 1, the pupil was observed to contract decidedly, but as the image became clear it again expanded to its original size. Again, at the moment of obtaining the second image the pupil contracted still more strongly, but as soon as the image became clear it again expanded nearly, if not entirely, to its original size. The same phenomena were observed for each of the images, only that in the nearest images, when the convergence of the optic axes was extreme and the first contraction very great, the pupil did not return entirely to its original dimensions.

I then made similar experiments on the image beyond the • Proc. Roy. Soc. April 1864.

+ Donders, "Accommodation and Refraction of the Eye," Transactions, p. 574.

real object. As before, I looked intently first on the real object at the distance of twelve inches until the pupil became steady. So soon as I gazed beyond the object the pupil of course expanded; but as soon as the image became clear, it again contracted to nearly its original size. In this last experiment the pupil is apt to be unsteady. This might have been expected; for, as we have already said, it is much more difficult to obtain this image clear, or to retain it when obtained.

There is no doubt of the fact, therefore, that the contraction of the pupil is most intimately associated with the focal adjustment.

I believe that this principle of dissociation of consensual adjustments explains perfectly certain phenomena of the stereoscope. It is well known that many persons experience difficulty in seeing stereoscopic pictures distinctly even when the two pictures are brought into perfect coincidence; and I believe all persons experience some fatigue to the eyes in looking at stereoscopic pictures for a considerable length of time. I have often felt both the difficulty and the fatigue, though to a much less degree than most persons. The explanation of this difficulty is as follows. We judge of distance, as is well known, by the axial adjustment. If, then, the two pictures are so taken that, in order to bring them together, the visual lines must meet at a certain distance, say, fifty yards, then the picture will be seen at that distance, and of course very much enlarged. But in order to see the picture clearly, the rays must come to the eye as if they diverged from the same distance; for the eyes are adjusted for that distance. To fulfil this condition lenses are always used; but it is obvious that a given pair of lenses are suitable for one distance only. For all other distances or degrees of optic convergence there must be some degree of dissociation of the two adjustments; and this is both difficult and fatiguing to most persons.

I have found that observations upon the images of the ruled diagram are a most delicate means of determining both the rotations of the eye and the position of the horopter. I hope in my next communication to take up this most difficult subject.

[To be continued.]

XIX. Notices respecting New Books.

Elementary Lessons in Astronomy. By J. NORMAN LOCK YER, F. R.A.S. London and Cambridge: Macmillan and Co.

THE

HE schools of our country are much indebted to the author of this little work both for the substance and the shape of the information which he has given them.

The amount of thought which he has bestowed upon the arrangement of his materials has not been thrown away, but has produced a work which will give the young student (as well as children of a larger growth) a clear and more complete idea of that great whole called the universe than most works of greater pretensions.

The custom hitherto has been for writers on astronomy to direct their readers' attention rather to the instruments by which observations are made, and to the principles according to which they are discussed, than to invite them to begin by taking a bird's-eye view of the Cosmos. We are told how to adjust a transit, and how to measure the sun's distance from the earth; but our energies are so much used up in understanding these things, that we have little strength left to contemplate as a whole the grand reality which they disclose.

But our author adopts a different method, and beginning with what we see, and first of all with the stars, we have a series of lessons in which the reader has clearly put before him a view of the magnitudes and distances of these bodies, as well as an account of their occasional peculiarities, such as colour and variability. In the nebular hypothesis, which is then described, we receive a hint of the process by which matter has been wrought from the state of primeval chaos into a sun or star.

When the reader has by this means become properly impressed with the magnitudes with which we deal in astronomy, one particular star is singled out for especial consideration. Our own star or sun is that one of all the host of heaven with which we are most intimately acquainted. His appearance and habits are therefore described, and we receive an insight into his chemical constitution.

Still proceeding downwards from greater to lesser magnitudes, we are next invited to consider the minor bodies of the solar system; and just as the sun was singled out as the type of the stars, so the earth is singled out as the type of the planets. Astronomers have been fond of drawing attention to the adaptation implied in the fact that the gravitating centres of the various systems are also the centres of light and heat; but it is only of late years that we have come to recognize that both these facts can be explained by the operation in two different ways of one and the same law. Accordingly we have another definition of planets, and one to which the author has given considerable prominence, namely that planets are cold while suns are hot, just as truly as that they are wandering while suns are fixed.

In clearness of diction, in comprehensiveness, in beauty of illustration this little volume is all that can be desired. In Lesson XLIV. we have an extremely useful summary of the methods by which the true positions of the heavenly bodies are obtained; and at the end of the volume we have a very complete and withal extremely intelligible account of the law of gravitation.

November 19, 1868.-Lieut.-General Sabine, President, in the

Chair.

read :

THE following communications were dut. John Herschel, R.E.

I have had two or three opportunities of seeing this spectrum to advantage of late. The storms at the period of the setting in of the south-west monsoon here are very frequent, and supply for a time almost incessant flashes, many of which are of course very brilliant. The first time I examined the light in the spectroscope I had no idea of measuring, but was content to realize the principal facts of a continuous spectrum crossed by bright lines; but subsequently I made several attempts (with some success) to obtain measures. That I was unable to do more in this line is due partly to the difficulty of utilizing the short-lived appearance, partly to that fascination of waiting for one more bright flash to verify the intersection, which can only be thoroughly appreciated by the aid of a similar experience.

66

The principal features of the spectrum are a more or less bright continuous spectrum crossed by numerous bright lines, so numerous indeed as to perplex one as to their identity. This perplexity is increased by the constantly changing appearance due to a variable illuminating-power. This variable character of the appearances is unquestionably the peculiar feature of the spectrum. It is not that the whole spectrum varies in brightness in the same degree, but that the relative intensities are variable, not only among the various lines, but between these and the continuous spectrum. The latter is sometimes very brilliant; and when that is the case, the red portion is very striking, though in general the spectrum seems to end abruptly at D40·34 (E=D+1.38, Kirchhoff's 120·7=D+0·55).

There is one principal line which I found equal to D+2.20 as the result of five independent measures. The probable error of this value is about 02. The general mean of all my measures of the principal nebular line (obtained from twelve different nebula) is 2.18, with a probable error of about 02. I have therefore very little doubt that these are the same, viz. the nitrogen line identified in the case of nebula by Mr. Huggins. This line in the lightning spectrum is narrow and sharply defined, and is conspicuously the brightest, except as noted below.

The next in prominence is situated about D+358 (F=D+2·73, Kirchhoff's 232·5=D+3.50). It is broader and less vivid, and not so well defined at the edges.

There are several other conspicuous lines, but none comparable to the first. I noticed a sharp line in the red, but did not get a

measure.

I said that at D+0.34 the continuous spectrum ends abruptly. A faint continuation, however, is frequently seen in bright flashes, very bright ones bringing out a brilliant red end crossed by a bright line.

The whole of the ordinary spectrum seems green and blue, or rather greenish blue; but as the usual prismatic order of colours is recognizable in bright flashes, it is to be inferred that the region from E to F is so much brighter as to give the character in question. What strikes one most, however, is the varying relative brightness of the continuous and linear spectra; sometimes the lines are scarcely seen, and sometimes very little else is seen. This may be nothing more than an illusion; but in the absence of any certainty that it is so, the impression left on the mind is worth recording.

The difficulty of discriminating between the many less prominent lines is immensely increased by the momentary character of the phenomenon. Before the mind has selected an individual, the feeble impression on the retina has vanished; and before another flash succeeds, the memory of the half-formed choice has vanished with it, and there is nothing on which to found a selection. Otherwise it would be easy enough to measure many more lines.

"Notice of an Observation of the Spectrum of a Solar Prominence." By J. N. Lockyer, Esq., in a Letter to the Secretary.

October 20, 1868.

SIR,-I beg to anticipate a more detailed communication by informing you that, after a number of failures, which made the attempt seem hopeless, I have this morning perfectly succeeded in obtaining and observing part of the spectrum of a solar prominence.

As a result I have established the existence of three bright lines in the following positions :

I. Absolutely coincident with C.

II. Nearly coincident with F.

III. Near D.

The third line (the one near D) is more refrangible than the more refrangible of the two darkest lines, by eight or nine degrees of Kirchhoff's scale. I cannot speak with exactness, as this part of the spectrum requires remapping.

I have evidence that the prominence was a very fine one.

The instrument employed is the solar spectroscope, the funds for the construction of which were supplied by the Government-Grant Committee. It is to be regretted that its construction has been so long delayed.

I have, &c.,

The Secretary of the Royal Society.

J. NORMAN LOCKYER.