cumstances that help the eye to measure the distance of an object must be reckoned the degree of clearness or faintness of the impression made by the object. Other things being the same, the clearer the impression, the nearer seems the object. Again, our visual estimate of the distance of an object is influenced by the presence or absence of other objects before and behind it with whose distance it may be compared. Thus a ship at sea looks too near, because there are no intervening objects to "throw it back," while the moon when near the horizon looks farther away (and consequently larger) than when over our heads, because of the numerous objects coming between the spectator and that luminary in the former case. Conversely, an object appears to be nearer the spectator when seen to be in front of another object than when there is no more remote object behind it. To each To each of these circumstances affecting the visual judgment of distance there corresponds an influence in the estimation of the remoteness of events. That is to say, our estimate of distance in past time is governed by two chief considerations, the vividness or faintness of the reviving image, and the recognized relation of the particular event to other events in front of and behind it. And this being so, we are liable to illusions of memory either when the degree of vividness deviates from the customary or normal degree for that particular distance, or when the mnemonic object wants its proper relations of before and after to other events. Our illusions of perspective commonly arise from a combination of both of these influences. Still they can, to some extent, be treated apart. Let us then first look at the influence of the circumstance of unusual vividness or faintness of the memorial image.

Speaking roughly and generally, we may say that the vividness of an image of memory decreases in proportion to the distance of the event. But this is not an infallible criterion of distance. The very fact that different people so often dispute about the dates and the relative order of past events experienced in common, shows pretty plainly that images of the same age tend to arise in the mind with very unequal degrees of vividness. Sometimes images of very remote inci

dents may start up in our minds with a singular degree of brightness and force. And when this is the case there is a disposition to think of them as near. If the relations of the event to other events preceding and succeeding it are not remembered, this momentary illusion will persist. We have all heard persons exclaim, "It seems only yesterday," under the sense of nearness which accompanies a recollection of a remote event when vividly excited. In passing from place to place, in talking with others, and in reading, we are liable to the sudden return by hidden paths of association of incidents that had long seemed forgotten, and when they thus start up fresh and sound, away from their proper surroundings, they invariably induce a feeling of their propinquity. No one can say why these particular images, long buried in oblivion, should thus resuscitate in possession of so much vitality. There seems indeed to be almost as much of the arbitrary and capricious in the selection by memory of its vivid images as in the selection of its images as a whole; and, this being so, it is plain that we are greatly exposed to the risk of illusion from this source.

It may be added that this appearance of nearness is greatly intensified by a conscious concentration of mind on the remote past. When, for example, old friends come together and talk over the days of yore, there is a gradual reinstatement of the lost experience. Incident after incident returns, adding something to the whole picture till it acquires a degree of completeness, coherence, and vividness that render it hardly distinguishable from a very recent experience. The process is like looking at a distant object through a field-glass. Mistiness disappears, fresh details come into view, till we seem to ourselves to be almost within reach of the object.

There is an opposite effect in the case of recent occurrences that, for some reason or another, have left but a faint impression on the memory; though this fact is not, perhaps, so familiar as the other. I met a friend, we will suppose, a few days since at my club, and we exchanged a few words. My mind was somewhat preoccupied at the time, and the occurrence did not stamp itself on my recollection. To-day I meet him

again, and he reminds me of a promise I made him at the time. His reminder suffices to restore a dim image of the incident, but the fact of its dimness leads to the illusion that it really happened much longer ago, and it is only on my friend's strong assurances, and on reflecting from other data that it must have occurred the day he mentions, that I am able to dismiss the illusion.

As we have hinted, in ordinary circumstances the tendency to illusion arising from too great or too little vividness in the memorial image is corrected by a recognition of the relation of the event to other events whose distance is supposed to be known. In point of fact, our conception of the past is made up of images of certain fixed prominent objects or landmarks, so to speak, in the memorial vista. And the full and distinct recollection of an event means the reference

of it to some one of these leading surviving images, these marking-posts which have not been wholly submerged by the waters of Lethe; the localization of it in some one of the great divisions of the past; and the recognition of its relations of antecedence and sequence to other well-known events. This operation is greatly aided by a comparison of our experiences with those of others, and by the employment of a simple common scheme of time-divisions, as years, seasons, months, etc. Indeed a past experience only takes up its right place, and appears at a correct distance from the present, when the mind, either alone, or still better in conjunction with other minds, has gone back on it again and again, noting its relations, and quietly putting it away, so to speak, into its proper niche in the ever-growing edifice of our fulfilled life.

This being so, what will happen when this process of localizing impressions in the past has not taken place, and when ideas of events return without any discoverable relations to other events? Clearly there will be a certainty of error. Not only will any unusual degree of vividness now take effect in producing an illusion of nearness, but the very fact of the absence of proper points of measurement with which the remembered event might be seen to be connected, will beget an erroneous idea of its distance. This brings us to the second circumstance in

the estimation of the degree of remoteness of past events.

The most striking examples of this illusory effect of detachment from fixed determining landmarks in the past are afforded by public events which lie outside the narrower circle of our personal life, and do not in the natural course of things become linked to any definitely localized points in the field of memory. These events are very stirring and engrossing for the time, but in many cases they pass out of the mind just as suddenly as they entered it. We have no occasion to revert to them, and if by chance we are afterward reminded of them, they are pretty certain to look too near. One reason of this is that the fact of their having greatly interested us has served to render their images particularly vivid. Another reason is that they reappear to us out of their true place, not distinctly projected behind a long series of intervening occurrences, and not seen as simultaneous or closely connected in the order of succession with other events known to be at a particular distance. A curious instance of this illusory effect was supplied not long since by the case of the ex-detectives, the expiration of whose term of punishment (three years) served as an occasion for the newspapers to recall the event of their trial and conviction. The news that three years had elapsed since this well-remembered occurrence proved very startling to ourselves, and to a number of our friends, all of us agreeing that the event did not seem to be at more than a third of its real distance. The newspapers themselves commented on the apparent rapidity of the time, and this shows pretty plainly that there was some cause at work producing a common illusion.

In

It is to be added that even when past events are properly attached to those that precede and succeed them, fluctuations may arise in the estimation of their distance in consequence of variations in the character of the intervening spaces. other words, our sense of distance in time is influenced by our sense of duration. According to Mr. G. J. Romanes, who has written a very interesting essay on our Consciousness of Time" (Mind, July, 1878), the sense of duration depends on two conditions. Time seems long, either when it is crowded with new

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and exciting impressions, as on a summer tour, or when we are dwelling on the fact of its passing, and so are particularly conscious of it, as in waiting for a train. It seems short when it is comparatively empty of exciting experiences, provided that the mind does not dwell on its passage. Thus it appears comparatively short when we are busily employed about our ordinary vocations. Hence it happens that events just preceding a time of unusual excitement and novel experience look farther off than events separated by the same interval of comparatively quiet impressions. Last Sunday's sermon seems much farther away after a week in Switzerland than after a week of customary experience at home. Not only does an event appear to take up different distances because separated from the present by seemingly unequal intervals; the very fact that the one interval has been filled with exciting impressions, the other with comparatively quiet ones, serves to give the mental image of the event a different degree of vividness and distinctness in the two cases. Our mental development is not only a process of retention of the old, it is a process of displacement of the old by the new. The more interesting or the more exciting the new, the more rapidly does the old tend to disappear. Hence the apparent extrusion of a recollection by supervening experiences of unusual impressiveness. Even dream-consciousness seems capable of effecting this result, for the incidents of the preceding evening often appear to be dimmed and thrust farther away after a night of exciting dreams. To young children events a year old look much farther off than to adults, just because the interval, full of novel impressions from its young world, appears to swell out, and because the very impressiveness and fascination of these experiences tend to obscure the earlier ones and to banish them farther from the present.

Enough has perhaps been said to show how much of uncertainty and of selfdeception enters into the processes of memory. This much-esteemed faculty, valuable and indispensable though it certainly is, can lay no claim to that absolute infallibility which is sometimes said to belong to it. Our individual recollection left to itself is liable to a number of illusions even with regard to fairly re

cent events, and in the case of remote ones it may be said to err habitually and uniformly in a greater or less degree. To speak plainly, we can never be certain on the ground of our personal recollection alone that a distant event happened exactly in the way and at the time that we suppose. Nor does there seem to be any simple way by mere reflection on the contents of our memory of distinguishing what kinds of recollection are likely to be illusory.

How then, it may be asked, can we ever be certain that we are faithfully recalling the actual events of the past? Given a fairly good, that is, a cultivated memory, it may be said that in the case of very recent events we may feel pretty certain that, when the conditions of careful attention at the time were present, a distinct recollection is substantially correct. Also it is obvious that with respect to all repeated experiences our memories afford practically safe guides. When memory becomes the basis of general knowledge, as of the truth that the pain of indigestion has followed a too copious indulgence in rich food, there is little room for an error of memory properly so called. On the other hand, when an event is not repeated in our experience, but forms a unique link in our personal history, the chances of error increase with the distance of the event, and here the best of us will do well to have resort to a process of verification and, if necessary, of correction. That is to say, we must look beyond our own internal mental states to some external facts. Thus the recollections of our early life may often be tested by letters written by ourselves or our friends at the time, by diaries, and so on. When there is no unerring objective record to be found, we may have recourse to the less satisfactory method of comparing our recollections with those of others. By so doing we may reach a rough average recollection which shall at least be free from any personal error. But even thus we cannot be sure of eliminating all error, since there may be a cause of illusion acting on all our minds alike, as, for example, the extraordinary nature of the occurrence, which would pretty certainly lead to a common exaggeration of its magnitude, etc., and since, moreover, this process of comparing recollections affords a

fine opportunity for that reading back a present preconception into the past, to which reference has already been made. The result of our inquiry is less alarming than it looks at first sight. Knowledge is valuable for action, and error is chiefly hurtful in so far as it misdirects conduct. Now, in a general way, we do not need to act upon a recollection of remote single events; our conduct is sufficiently shaped by an accurate recollection of recent single events, together

with those bundles of recollections of recurring events and sequences of events which constitute our knowledge of ourselves and our common knowledge of the world about us. Nature has done commendably well in endowing us with the means of cultivating our memories up to this point, and we ought not to blame her for not giving us powers which would only very rarely prove of any appreciable practical service to us.-Cornhill Magazine.

THE world of science and the world of fashion are so far removed from each other that they are seldom stirred by the same event, but the production of artificial diamonds has lately startled both these distant realms.

Mr. Hannay, of Glasgow, has recently exhibited before the Royal Society certain crystals which are no accidental productions, but direct results of a process conceived for a definite end. These have been examined by analysts like Professors Maskelyne, Roscoe, and Dewar, and declared to exhibit all the physical and chemical properties of true diamonds.

Mr. Hannay's gems are very small; but whether he will hereafter succeed in producing large stones, and what effect success of this kind would have on the value of the diamond, we do not propose to inquire. This is a question which concerns the world of fashion alone; the world of science is interested in asking by what means the crystallization of carbon has at length been accomplished.

Every one is acquainted with the various forms of the substance called carbon. It constitutes a large proportion of all animal and vegetable structures, and we know it best in an impure condition as coke or charcoal; but it occurs crystallized, and in a state of purity, in two very different forms, viz., diamond and plumbago, or black-lead.

Those bodies which resist all attempts of the chemist to resolve them into simpler forms of matter are called elements, and among the vast number of substances composing our earth, some sixty-four, which are for the most part

metals, are simple bodies; of these carbon is one.

Almost every substance which is capable of existing in the solid state assumes, under favorable conditions, a distinct geometrical figure. This power which bodies possess of taking on definite forms is called crystallization, and its most beautiful examples are found among natural minerals, the results of exceedingly slow changes occurring in the substance within the earth. Artificial crystals may be obtained from solutions, by fusion, and in the passage of bodies from the gaseous to the solid condition. Thus crystals of common salt are formed by the evaporation of brine; many metals, as iron and bismuth, crystallize on cooling after being melted; and the vapors of some substances, like iodine for example, deposit crystals in the act of condensation.

Everybody possesses its own distinct crystalline form; every crystal is a geometrical figure, usually bounded by plane surfaces having angles of constant value, and the science of crystallography teaches us to distinguish substances by the measurement of these angles. It is invariably found that artificial crystals which have been deposited slowly and quietly surpass in size, regularity, and beauty those of more rapid formation; hence it is conjectured that natural minerals owe their great perfection to very gradual deposition in the rocks within. which they are found.

Under different conditions the same substance sometimes assumes two crystalline forms, of which somewhat uncommon phenomenon carbon furnishes an example by crystallizing now into

diamond, and now into graphite or plumbago.

Although found in every quarter of the globe, the diamond is the rarest as it is the hardest known mineral. It occurs exclusively among gold-bearing rocks, or sands derived from gold-bearing rocks, and among strata, which, though originally soft shaly deposits of sand or mud, have been metamorphosed," as it is called, into hard crystalline schists. It was once supposed by geologists that the metamorphic rocks were deposited in their existing crystalline form from a boiling ocean enveloping the still heated globe; but it is now known that these formations were originally deposited as mud or sand, and have been transmuted into schists by the influence of subterranean heat acting under great pressure, through lengthened periods of time, and aided by thermal water or steam permeating the porous rocks and giving rise to various chemical decompositions and new combinations within them. The diamond probably originates, like coal or mineral oil, from the gradual decomposition of vegetable or animal matter; and we may therefore regard the brilliants which we prize in the drawing-room as having been slowly elaborated from carbonaceous matter furnished by some dead fish, or rotting plant, originally buried in the mud of an inconceivably ancient paleozoic shore.

It will now be seen that, in order to produce the diamond artificially, some means must first be devised whereby the element carbon, which will dissolve in no liquid and vaporize in no flame, can be rendered soluble or gaseous, from either of which conditions it might then probably be recovered in a crystalline form, as happens in the case of other bodies.

Mr. Hannay's attempts to crystallize carbon originated from a research of a very different character. Water, as we all know, vaporizes at a heat of 212° Fahr., and in the same way every liquid has its "boiling-point," or temperature at which it ceases to be a fluid and becomes a gas. Little is known about the condition of matter immediately beyond the "critical point," as the moment of passage from the liquid to the gaseous state is called; and while investigating

this subject it occurred to Mr. Hannay that some insight might be gained into a state of things then so obscure as to be thought hopeless, by dissolving in the liquid under examination some solid substance which fused at a temperature much above the critical point of the fluid.

Sulphur, for example, melts at 111 Fahr., and is soluble in carbon di-oxide, a liquid whose boiling-point is 42°. When such a solution was vaporized it was found that the sulphur was not deposited, but remained diffused in the atmosphere of di-oxide vapor; or, in other words, the sulphur was dissolved in the gas. If the side of a tube containing such a gaseous solution of sulphur is approached by a red-hot iron, the part next the source of heat becomes coated with a crystalline deposit, which redissolves into the gas on the removal of the heat. In the course of his experiments on the solubility of solids in gases Mr. Hannay further noticed that many bodies, such as alumina and silica, which, like carbon, are insoluble in water, dissolved to a considerable extent in water-gas, or water at the critical point when it is neither a true liquid nor a true gas. This fact suggested to him that a solvent might even be found for the hitherto insoluble element, carbon ; and as gaseous solutions were found to yield crystalline solids in almost every case upon the withdrawal or dilution of the solvent gas, it was hoped that from such a gaseous solution of carbon crystals of diamond might be obtained.

After a large number of experiments, however, it was found that neither charcoal, lampblack, nor black-lead would dissolve in the most probable solvents when these were brought to their critical points, and a new road out of the difficulty had accordingly to be sought.

Chemists have long known that what is called the "nascent" state of matter is one very favorable to chemical combination. Thus nitrogen, for example, refuses to combine with hydrogen, but if these two substances are simultaneously liberated from some previous combination they unite at the moment of birth with the utmost ease. Bearing this in mind, it was ascertained that when a gas containing both carbon and hydrogen is heated under pressure in presence of a metal, the hydrogen is at