Mr. Proctor has advanced, however, a very ingeniouslycontrived hypothesis to account for the phenomena observed, and which he regards as being probably the effects of contrast, and this view he has backed by some sound arguments. Mr. Proctor would seek the explanation of the variation in the floor of Plato in the same source as that of the wellknown phenomena of a dark body looking still darker when on a brighter background. Thus at sunrise the floor of Plato is thrown against a dark background, due to the sombre, barely illuminated, surrounding regions, whilst at full moon it has for a background the brilliantly illuminated surrounding highlands, and should look much darker. Thus ("Quarterly Journal of Science," vol. xl., p. 504, Oct., 1873) Mr. Proctor says-" Setting aside other possible explanations . . . there is the effect of contrast to be considered. This I believe, from my own observations, to afford the true explanation of the observed phenomena. Plato lies on lunar highlands, which shine very brilliantly under high solar illumination. Towards the Mare Imbrium a comparatively narrow ridge separates the floor from that region. Now when the terminator has just passed beyond Plato, the surrounding wall is not nearly so bright as at the time of full moon; the black shadow of its western ridge occupies the western side of the floor; and the eye, in estimating the tint of Plato, is neither oppressed with the glare of general light on the one hand, nor forced to compare the tint of the floor directly and solely with a much brighter surface; if the comparison is made on the east, with an illuminated wall, it is made on the west with a perfectly black shadow-streak. Similar remarks apply to the time when the terminator is about to pass away from Plato. But at the time of full moon the highlands around Plato are very brilliantly illuminated. The glare necessarily makes Plato itself look relatively dark, notwithstanding the fact that the floor is also much more brilliantly illuminated; for it is a recognised fact, that surfaces of unequal light-reflecting capacity appear to differ more in brightness under a high illumination than when they are only faintly illuminated. We know, in fact, that a surface which is only dark looks almost or perfectly black when itself and a brighter background are under strong illumination." Mr. Proctor proceeds to illustrate this by reference to the fact that occasionally some of Jupiter's satellites look bright when seen against the sky, and yet dark when seen against the bright surface of the planet when in transit, and then continues"Such an observation as this appears to me to be decisive

against mere eye-estimations, showing that absolutely no reliance can be placed on them unless some contrivance is employed to destroy the effect of contrast. As it is conceded that none of the observed darkenings of the floor of Plato have been other than eye-estimations, no further discussion seems needed, or can in my opinion be legitimately given to the subject. I may mention, however, that having, though as yet in an imperfect manner, studied Plato with a much reduced field, so that I could eliminate to some degree the effects of contrast, I have not found that the floor grows relatively darker towards the time of full moon."

This most ingenious explanation devised by Mr. Proctor to account for the phenomena presented by the floor of Plato, and by assuming the correctness of which he is enabled to draw the very strong conclusions just quoted, is not an explanation which will be for one moment admitted by astronomers who are familiar with the phenomena presented by the various lunar formations, for they at once perceive numerous and apparently fatal objections to it, when applied as Mr. Proctor proposes to do its very nature, in fact, suggests the idea that its able author overlooked the relationship in which Plato and the anomalous darkening of its floor stood to other similar formations with the normal behaviour of their interiors, for his hypothesis supposes nothing exceptional in the constitution of the walled plain Plato. If, then, the great darkening observed to occur in the tint of the interior of Plato is merely apparent, and only what must occur when a darkish walled plain is surrounded by a bright background, or rather bright environs, and this is all Mr. Proctor ascribes to it, it must be a perfectly normal occurrence, and the same must take place in every similarly placed formation, unless that has something anomalous about it to prevent this taking place. But all selenographers could instance a number of such walled plains where no such darkening occurs. Are we, then, to assume that all these possess anomalously constituted interiors, and that only Plato, of all the lunar formations, exhibits the normal phenomena. This is, of course, entirely inadmissible, and selenographers are thoroughly aware that the effects of contrast alluded to by Mr. Proctor are entirely incapable of bringing about such an immense darkening in tint as is apparent in the case of the floor of Plato.

As an instance illustrating this, the behaviour of the walled plain named Archimedes may be considered in connection with that of Plato. This walled plain lies not very far to the south of Plato, and is, if slightly smaller, of the

same form, and bordered by mountains of the same height and brightness; whilst lying on the open plain it is environed by a surface little inferior in brightness to that around Plato. At the period when the anomalous darkening of the floor of Plato commences, the interiors of the two formations are sensibly of the same tint, a cold light yellow grey, both being brighter than the surface of the intervening Mare Imbrium. But while the interior of Plato darkens until it is almost black, the floor of the similar formation Archimedes grows gradually lighter until it is a bright yellowish grey, thus following the same course as nearly every other lunar formation. If, then, the darkening presented by the floor of Plato is, as Mr. Proctor contends, the natural result of the contrast between its surface and its bright environs, why does not Archimedes, similarly placed, and with a floor at one period of similar darkness, present the same phenomena ? The fact that Archimedes and the many other similarly placed formations do not present this darkening, and that Plato alone of the many hundred lunar-walled plains does darken to this great extent, shows that there is something anomalous in the constitution of its interior, which is what selenographers contend. It is obvious, therefore, that the hypothesis framed by Mr. Proctor to account for the phenomena fails to do so when critically examined.

In his paper, Mr. Proctor refers, however, to what he considers direct evidence that the darkening of the floor of Plato is a mere effect of contrast, namely, that when examined with a small field of view, so as to shut out the greater portion of the surrounding bright environs, the interior of the walled plain showed no darkening towards full. This observation of Mr. Proctor's, if confirmed, would have been extremely important, because it would establish the fact of the darkening of the interior of Plato being merely the effect of contrast, and so make the general behaviour of the similarly situated walled plains the real anomaly. The observation of Mr. Proctor was confessedly imperfect, but the point required careful examination. For this purpose a numerous series of observations, with the field of view of the telescope purposely constructed to small dimensions, was undertaken, in some the field being reduced so small that only a portion even of the interior of Plato could be seen at once, so that the entire environs were shut out; in others, the border of the walled plain was included; and in others, more or less of the surrounding brighter regions in all less different sized fields were employed, a revolving diaphragm with apertures of different sizes being employed.

With these means a series of several thousand observations, extending over a period of nearly two years, was made, and not only of Plato, but of similarly placed or coloured formations. Nothing new, however, resulted; the interior of Plato went through the same course of darkening when observed through small fields of view as before, and the other regions exhibited their usual course of variations. The observation of Mr. Proctor was not therefore confirmed.

For the purpose of still more decisively settling this point, a piece of apparatus was constructed whereby-through the agency of shifting slides of blackened metal, with different sized apertures-any two or three different small regions of the moon could be seen isolated in the same field, all other portions of the surface being cut out. Moreover, by illuminating the upper side of the dark shutters the great contrast between the bright moon and a dark field could be subdued. In any case, however, as each of the two or three portions were seen at once, under exactly the same conditions, the effect of contrast was removed. By this means, from time to time, the floor of Plato was compared with other small portions of the lunar surface, so that any change would be at once detectable, and a numerous series of observations made. Under these conditions the darkening in the floor of Plato was as marked as ever, though all the effects of contrast with the surrounding bright highlands were entirely removed, for these bright highlands were shut out of view; and as both portions of the moon were thus placed under exactly similar conditions were they of the same brightness they should have so appeared when thus directly compared. By this means the reality of the darkening of the floor of Plato was incontestibly established, and shown to remain unaffected when the effects of contrast are thus eliminated.*

The explanation advanced by Mr. Proctor to account for

Since this was written a statement has appeared in a letter to an English periodical, from Mr. Proctor, to the effect that, whilst somewhere in America, some American gentleman-neither whose address nor name is mentioned-once told him that he thought no real darkening in the floor of Plato tock place. This must be taken quantum valeat, and is only mentioned so that all published information on this subject may be referred to. A reviewer of my work on the Moon, in a very curt and gauche manner, contradicts the above statement, without giving any grounds for his behaviour. It is to be presumed he drew on himself for the numerous series of observations necessary to authorise a point blank denial of a result established by a long series of elaborate observations made with powerful instruments. Until these are published, however, and nothing is known of them,-no more need be said of such a contradiction.

the darkening of the floor of Plato is not therefore adequate for its purpose. Unquestionably it may have some effect, but whatever its amount may be it is too small to be sensible. Some small variations in the amount of the darkening of Plato, when observed by the various methods detailed, were also found to exist, but they were relatively very small in amount. These differences were, however, no more than were to be expected.

What, then, is the cause of this singular darkening in the floor of Plato? What reason can exist why that, as the solar altitude increases, the interior of Plato-unlike any other similar formation upon the surface of the moonshould alter in colour and brightness from a cold yellow-grey to a deep steel-grey, or almost black tint? It arises evidently, from what has been stated, from the anomalous conditions prevailing upon the surface of the interior of the wall plain. It is extremely improbable that these anomalous conditions should be an exceptional and fanciful configuration of the surface, and it is more natural to suppose that they exist in reference either to the condition or constitution of the surface. But however successfully the subject may have been pursued so far, here all further progress in elucidating this question is suddenly arrested, no matériel exist for a further investigation. The observations now required are not those of simple observation with the usual astronomical appliances; what is wanted is special observations with special appliances, and these are at present not to be obtained. And, in fact, the great difficulties in the way of making these observations, and the powerful and costly instruments, and the experience required, effectually debar all except a few favoured astronomers from pursuing these observations. Thus, as in so many selenographical problems, patient observation establishes the existence of certain phenomena, but the elucidation of the meaning of the phenomena established is checked for want of special observations that are never made. For these selenographers have to appeal to those astronomers devoted to what has been termed astronomical physics, but they are too much engaged on, to them, more fascinating subjects to be able to assist selenographers.

Although, therefore, the means do not exist for saying, in one manner or another, this change results from this cause or from that cause, it is not fruitless to enquire what their studies have led selenographers to believe to be the cause of this change. The opinion, then, that appears to agree with the views of the most experienced selenographers who have