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with regard to its successful delineation. Much attention has of late been given to selenography, and very deservedly; but the practical results might be of more value, if operations were conducted on a more systematic plan, and with a more definite understanding of the possible causes of misconception or illusion, such as might easily influence the judgment of an inexperienced, although keen-sighted and careful observer. A few hints, therefore, may be permitted, which may be of advantage to the student at the commencement of his labours.
It may be assumed that no fresh attempt will now be made to represent any extensive area of the lunar surface. This would be only (except for practice) so much trouble thrown away. The broader features have been already abundantly delineated by Schriiter, Lohrmann, and Beer and Maldler. Our immediate object is to fill in the outline, which they have given with so much general fidelity, but by no means a corresponding minuteness of detail. This, indeed, could not in fairness have been expected, and would naturally be left to the labours of the future. And now a single large walled-plain or crater, or insulated mountain-group, or definite portion of a level region, will provide sufficient occupation for many nights, till every detail which the telescope employed can grasp is identified and delineated with as much correctness as the present state of our knowledge may justly require. Much has been said of the value of photography in producing such representations. Nor is there the slightest intention of disparaging ' its just claims. Great honour is due to those eminent men who have carried it through such discouraging impediments: its results have already proved most valuable, and its peculiar advantages are otherwise unattainable. N0 eye can be so certain of including every visible feature; no hand can place every detail so accurately in its relative position. The supe— riority of the self-produced picture in each of these respects will be very evident to anyone who has remarked how often objects are unnoticed in sketching, the detection of which on :a subsequent occasion may prove a source of error (Schroter’s new crater in Hevel was probably a case in point), or how much drawings of the same objects by different hands differ in proportion and character, of which the various representations of the walled-plain Gassendi are an instructive example. But there is a sharpness and decidedness in telescopic vision, which we miss in photographic portraits; and from this cause the camera may fail to exhibit the existence, or at any rate the true character, of the minutest features. And besides this, whatever may be the perfection of the photographic apparatus, the nights in which the air is sufficiently steady to admit of the development of its full ability are few indeed, compared
with those, so familiar to every experienced observer, in which hard, clear definition is combined with unsteadiness of image; the irregular refraction of the atmosphere producing the very same effect that the undulations of surface would on the position of an object seen, otherwise with great distinctness, in the bed of a running stream. In such a state of air~—the best oftentimes that an observer is fortunate enough to meet with in the course of many nights—photography would be confusion, while good drawing is perfectly practicable.
But then, it scarcely need be added, it ought to be good drawing. Not that rough sketches may not prove occasionally of value, when they contain clear evidence of some previously unnoticed object; but the student ought to be especially warned against a facile carelessness of execution, which must prove unsatisfactory to others, and ultimately to himself. And the professed artist is perhaps especially likely to be thus betrayed into error. Of course,'before he sets to work upon any celestial object, he will have laid aside all thought of making a pleasing picture; but he may still be misled from being habitually conversant with that essential principle of art, which teaches us that likeness is more connected with breadth of effect, than with elaboration of detail. In the present case, however, it is on accurate minuteness that we have to depend for our hope of discovery; experience contravenes all reasonable expectation of physical change on a great scale in our own days, and our knowledge of the real nature of the lunar surface, and any operations that may be in progress there, must depend on the faithful representation of the smallest objects that our telescopes are capable of showing with distinctness.
The principal requisites of good drawing, as applied to the lunar surface, may be easily specified. They are (1) correctness of form. It would seem almost superfluous to specify this, but for the glaring inaccuracy in this respect, which is occasionally apparent even in drawings which ought, from their advantageous circumstances, to take a high rank. And it is here that a knowledge of effect and perspective becomes of great value. Although we demand a reSOIVed fidelity, which is determined to represent everything just as it appears, however anomalous that appearance may be, and although nothing could easily be worse than an attempt to force a delineation into accordance with the rules of art, yet it is certain‘that a previous apprehension of the real nature of what is seen will often give material aid in preventing mistakes and facilitating observation. When, for instance, we know that every circle lying in a certain direction as regards the line of sight_must be projected into an ellipse of known proportion and position,
we shall find it far more easy to draw with fidelity an obliquely viewed landscape, in which circular forms prevail, as well as to estimate correctly the amount and direction of such deviations from perfect circularity, as will probably be found. But supposing correctness of form to be ensured, we have another point of equal or sometimes even greater importance to attend to, the relative proportion of size. The neglect of this has, as much as anything, retarded the solution of the interesting question whether volcanic activity is still in progress on the moon. The minute craters which abound in so many lunar landscapes have been often jotted down, as if the observer was quite satisfied with recording their existence, and attached little importance to their relative dimensions; yet this is the only criterion, as it is an equally easy and satisfactory one, of their activity or extinction. And (3) the relative position of objects should also be attended to. Less material than the two preceding, it must not be neglected, as an element in the required faithfulness of representation, and in some cases as avoiding considerable trouble in identification. And we may add here a caution against finishing off a careful design with a sketchy margin of general resemblance, which at some future time, when the limits of accuracy are no longer remembered, may prove a source of perplexity and annoyance. There is some latitude as to the materials which may be employed. Those who can handle a pencil dexterously may find it answer well; but they should not forget-to set the sketch afterwards. Delicate pen and ink drawing, in the manner of etching, may be made very effective, but is not easy of correction in case of mistake. With any one accustomed to the brush, watercolour lamp-black succeeds very well—not Indian ink, as it is troublesome to get it black enough for the deep shadows, and is too indelible in case of error. Professor Piazzi Smyth used .oil colour ; but this is less easily managed, excepting by a practised hand. Black and white chalk on a grey surface gives a striking effect, but independently of its great liability to damage, it requires an inconveniently large .scale, or with these materials the requisite minuteness will be unattainable. Probably the most unexceptionable mode (I regret that I cannot.speak of it from personal experience) would be the employment of tube-colours,consisting of black and white, and admitting of ready mixture, and correction where requisite, on a ground of neutral grey composed of the same materials. In this wayythere would be little difficulty in representing Ithose ‘half-tones,’ or gradations of shade, the want of which is so apparent in ordinary lunar drawings to an experienced eye. The most essential purposes may no doubt be secured without them, and their introduction requires a good deal of time and some skill. But if conveniently attainable, they would add materially both to the accuracy and the effect of” the resemblance. It is true that in consequence of the extreme tenuity of the lunar atmosphere, there is an entire absence of that diffusion and softening of light which so especially characterises the terrestrial landscape. With a daylight sky, resembling probably that of our midnight, and devoid of ' the mists of sunrise or sunset, every spot where the sun is not actually shining would of course be shrouded in intense blackness of shade; but there are abundance of banks and slopes lying in planes which intersect the solar disc, and consequently receive a partial or penumbral illumination, contributing greatly to the varied effect of the landscape, and to the truth and certainty of its relief. And the more fully these are exhibited in any mode of representation the better.
A very little eXperience in drawing will convince us of the expediency of confining ourselves to a limited area. It is very desirable not merely to be able to complete what we have undertaken, but to avoid the change of effect inseparable from a delineation'protracted through many consecutive hours. The alteration in the amount of shadow, especially near the terminator, during a winter’s evening, would strike an unaccustomed observer with surprise.
With respect to the choice of regions suitable for representation, especially for the commencing student, those approaching to the limbs musttake decidedly the last place. N 0t only is ‘the perspective foreshortening likely to cause perplexity and mistake among unknown objects, but, owing to the neverceasing action of libration, that foreshortening is in a state of continual restlessness. Every night we look upon a fresh limb; a similar aspect is slow in its return; entire correspondence is not restored till after three years; and during the brief period of its continuance he must be a novice indeed in actual work, who would flatter himself, at least in an English sky, with the hope of similar atmospheric conditions. Regions near the centre of the disc are far more suitable, especially for early efforts; but it must not be forgotten that even such districts are not altogether free from the displacement occasioned by libration, and its effect in changing the relative proportions and situations of objects ought to be studied and allowed for. In fact a clear understanding of this cause of' apparent variation must underlie any satisfactory investigation of the interesting enquiry already referred to, whether the convulsions which have in former ages shattered the superficies of the lunar globe have now sunk into entire quiescence, as well as that scarcely less curious question, whether physical change may not be detected, on or immediately above the
surface, in connection with the varying temperature of differ~ ent parts of the lunar day. Apparent variation may not unfrequently be perceived among minor details; but nothing satisfactory would result from the precipitate inferences which have been sometimes drawn from it, and a little further examination of this subject may be permitted, as it has not always been sufficiently considered in its full extent.
When there is evidence of want of correspondence in drawings made by ourselves 0r others at different dates, we have first to bear in mind the possibility of omission through de
‘ fective attention—a not' infrequent source of error, and one which is subsequently irremediable. Some observers, no doubt, are more liable to it than others, but none could claim absolute immunity ; to photography alone is reserved the triumph over this form of error. Then as regards the minutest visible details, the state of our own atmosphere. may exercise considerable influence. But besides these potent sources of discrepancy, others remain in reserve of a very different character. It need hardly be said that the true relief of any surface not viewed in profile is decided by the known and unalterable laws of light and shade. These laws indeed admit of no variation, but their application sometimes involves cases liable to misconstruction, from peculiarity of form in the projecting or receiving surfaces, and if the position of those surfaces is itself liable to change, either with regard to incident or reflected light, or both, it is evident how much the chances of some illusory effect may be multiplied. A few instances may be of use in the way of illustration.
No misapprehension could arise in the case of shadows projected from regular forms upon an uniform sphere. But it is needless to observe how far these conditions are from being fulfilled upon the moon; and not only are the shadows very irregular in their shape, but they are liable to many incidental Causes of disturbance, partly of a real, partly of an optical character. Independent of their normal variation in actual length according to the height of the sun above the lunar horizon, they may also vary in actual length from the unevenness of the recipient surface. For instance, a shadow will be gradually shortened, or suddenly blunted, if its extremity falls on a rising slope, or against a rapid elevation; or it will be correspondingly extended under opposite circumstances; and thus the edge of the shadow of a long precipice of uniform height, instead of being bounded by a straight line, as it would be on a plain, may become curved in various directions, or even jagged, if falling on very irregular ground. An apparent ' lengthening or shortening may also take place in obliquely viewed regions, when illuminated objects are brought by change