Imatges de pÓgina
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water for a much longer time than green peas-that in fact, the latter are reduced to a pulp in a space of time hardly sufficient to soften the former. Professor Tyndall 17 found that an infusion of old hay was much more difficult to sterilise than one of fresh hay, and that, while a few minutes' boiling sufficed to kill all germs in the latter, those contained in hay a year or two old resisted the action of heat for a very long time. He naturally concluded from this that the almost infinitely minute germs of microphytes may, in just the same manner as peas, become dried and hardened, and so able to oppose a long and obstinate resistance to the action of heat.

Assuming, then, as it seems one is bound to do, that germs may and do undergo this excessive induration, one is able to form some conception of the difficulty of sterilising a fluid in which, as is certainly the case in very many instances, such veteran germs occur, and to appreciate the ingenuity with which Professor Tyndall has overcome the difficulty. He finds 18 that frequent applications of a low degree of heat, applied at intervals, have a far greater sterilising effect than a single application of a very high temperature.

A given fluid may contain germs of all ages. If this fluid is boiled for a considerable period, all those of recent formation will be killed at once, while those of considerable age will only be just sufficiently softened to enable them to germinate subsequently. If, on the other hand, the fluid is first heated for a short time-and a fraction of a second is often sufficient-the recent germs will be killed, and those a degree older so softened, that, after a period of latency, they are ready to germinate. Heat now applied for a short time will kill these, and fit a third set for growth; and the same course may be adopted for successive crops, until even the hardest and driest germ is killed. It was found that a fluid which was not rendered barren by boiling for an hour was completely sterilised by this process, although never heated up to the boiling point, and although the whole time of heating did not amount to five minutes; and even the infusions which had given Professor Tyndall most trouble were, without exception, rendered permanently barren.

Many years ago an ingenious tale appeared in one of the magazines, the hero of which had a theory to the effect that the last object seen by a dying person was imprinted on the retina, and could, by suitable means, be photographed, and so preserved. His researches on this subject and his final success were detailed with great appearance of truth, and in the end he discovered the murderer of his sister by recognising in a chance-met stranger the original of the portrait

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Preliminary Note on the Development of Organisms in Organic Infusions.' Proceedings of the Royal Society, vol. xxv. No. 177.

18On Heat as a Germicide when discontinuously applied.' Royal Society in February.

Read before the

he had, years before, obtained from the eye of the victim. It is curious how prophetic this seemingly wild fable has turned out to be of a wonderful discovery made within the last few weeks.

A short time since Franz Boll observed that the retinas of all animals, instead of being white or greyish, as was supposed, were of a beautiful purple-red hue. Boll supposed that this colour was destroyed during life by strong light and restored by darkness, and that it invariably disappeared, for ever, a few seconds after death.

Since the publication of Boll's results, the subject has been investigated in great detail by Kühne,19 who has arrived at conclusions the importance of which can hardly be too highly estimated. He found, first of all, that although the sight-purple (Sehpurpur) disappears within half a minute after death in bright sunlight, yet that in gaslight it remains unaltered for twenty to thirty minutes, and in the dark, or when exposed only to the yellow light of the sodium flame, for twenty-four to forty-eight hours after the time, in fact, at which decomposition has set in. The colour, moreover, exists only in the layer of rods and cones, and although discharged by high temperature and by certain reagents, it remains unaffected by others, such as common salt, alum, and glycerine, and is also unaltered when the retina is spread on a glass plate-of course, in yellow light-and allowed to undergo complete desiccation. Furthermore, when a retina was spread out on glass, partly covered by strips of tinfoil, and then exposed to light, it was found that the otherwise bleached membrane retained its beautiful purple colour wherever it had been protected from the action of light by the tinfoil. In other words, there was impressed upon it a positive photograph of the strips.

It was now necessary to decide the question, How is the sightpurple renewed in the living animal after being bleached by light? The retina from one eye of a frog was removed and placed on a glass plate; an equatorial section was made of the other eye, and its posterior half was exposed to light, under the same conditions as the removed retina, until the latter was completely bleached. The second retina, still in its natural relations to the other coats of the eye, but presumably with its colour discharged, was then taken to the sodium chamber, removed, placed on glass, and again brought into ordinary daylight. The purple, colour was found to be perfectly restored. From another eye the retina was removed in such a way that some black fibres of the underlying choroid coat still adhered to it; it was then spread out on glass and exposed to light. The bleaching effect was less marked when the choroid was left. Still more instructive is an experiment in which a portion of the retina was removed from its natural position until bleached, and then carefully put back, so as to be once more in contact with the choroid;

19 Zur Photochemie der Netzhaut. Gelesen in der Sitzung des naturhistorischmedicinischen Vereins zu Heidelberg den 5 Januar, 1877.

when removed after a few minutes it was found that the sight-purple was completely renewed. It is thus proved that the restoration of the sensitive pigment is the special function of the choroid, the hexagonal cells of which, extending for a short distance between the rods and cones, continually sensitise the latter, as they become bleached by light.

It follows from this that, as Kühne observes in a subsequent paper,20 normal vision is only possible while a constant balance exists between the bleaching of the rods by light and the purpurogenous action of the retinal epithelium. If, therefore, this balance were destroyed by a prolonged exposure to light, it should be possible to obtain a permanent optograph of a luminous object; and this Kühne now set himself to accomplish, devoting his attention to the eyes of mammals, in which the purple-forming function of the choroid ceases a few minutes after death.

A rabbit was fixed at a short distance (1.5 metre) from a square hole, of 30 centimetres in the side, in a window-shutter; its head was covered for a short interval with a black cloth, the cloth was removed, and the eye exposed to the light of midday for three minutes. The animal was then instantly beheaded, the eye removed in a chamber lighted by the sodium flame, and placed in a solution of alum. On the second day the retina was removed, and was found to exhibit, on a rose-red ground, a white image about one square millimetre in size, almost quadrate in shape, and with its edges sharp as if drawn by a ruler!

Naturally Kühne was not satisfied with this single experiment, decisive as it was, but a week after its publication brought out a third communication, in which even more beautiful and astonishing results are described.21 A rabbit was treated in the same manner as the last, except that it was placed a short distance from an entire window, and not a hole in a shutter: in this case the whole image of the window was accurately photographed--the panes white, the crossbars red and sharply defined. It was found also, as might have been expected, that a better image was obtained from the eye of a rabbit just killed than from one actually living, it being difficult in the latter case to overcome the regenerating action of the choroid on the sight-purple.

Lastly, Kühne tried the simplest possible method of optography: the head of a rabbit was cut off, and, without any preparation, held for ten minutes under the middle of a large skylight. After the usual treatment with alum, the retina was examined, and on it was seen the perfectly sharp image of the skylight, with every pane and cross-bar accurately reproduced, and, at some distance, a smaller

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Vorläufige Mittheilung über optographische Versuche.' Centralblatt für die med. Wissenschaften, 1877, No. 3.

Zweite Mittheilung über Optographie.' Centralblatt, 1877, No. 4.

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image of the second skylight of the room, the light from which of course fell obliquely into the eye.

To summarise the essential conditions of vision are essentially photographic: the purple layer of rods and cones is altogether analogous to a sensitised plate, the colour of which is discharged by light, but, during life, immediately renewed by the layer of epithelial cells in contact with it. And thus a great stride has been made in bringing the mysterious processes of life within the grasp of ordinary chemico-physical laws. Much yet remains to be done; the realm of things settled is still but an

isle of bliss Midmost the beating of a steely sea;

and it will be a long time yet before the desirable, though perhaps somewhat dreary, state of things comes to pass, when the biologist may, according to his temperament, sit down and weep that he has no more worlds to conquer, or sing his Nunc dimittis at having no more problems to settle and no more battles to fight.

AN ACTOR'S NOTES ON SHAKSPEARE.

No. 2.

HAMLET AND OPHELIA.

ACT III. SCENE 1.

THE riddle of this scene may be read in many ways, and in attempting to give any idea of its interpretation I do so with all reverence and modesty, believing that an honest confession of one's thoughts on such a subject may be received at any rate with some interest, if not with assent.

The first point, and one upon which differences of stage business have often turned, is whether Hamlet knew throughout the scene that he was watched by the King and Polonius. It is all enacted under espial. The interview between Hamlet and Ophelia is contrived. Is he conscious of this from the outset, or does he discover it in the course of the scene?

The text tells us that he knew he was being watched from the first, for in the quartos of 1603 and 1604 (the complete play) Hamlet enters before the exeunt words of Polonius to the King-Let's withdraw, my lord'-catches sight of them as they retire into the study' (an ante-room leading from the hall), and evidently guesses them to be intent on eaves-dropping. We know, besides, that the King had sent for him, and that he was quite shrewd enough to suspect some design. No doubt his mind is much preoccupied-preoccupied, too, with sublimer matters, and unlikely to be greatly fretted for the moment by any incident which he would regard as despicable. His thoughts on death reveal the bent, the tension, and the gravity of the mood in which he is when he observes the significant withdrawal of the King and Polonius, but when aware of the presence of Ophelia all else is banished from his consciousness. It is therefore not only conceivable but probable that the circumstances which indicated a set design to spy upon him fell at first on an only half-awakened sense, and that the fact, though clear, but faintly lingered in his mind beneath the surface of his nobler contemplations. It is when he perceives the spies that he remembers he is watched.

The second point of interest is whether Ophelia knew that her father and the King were eaves-droppers. Superficially this may be assumed, and allowance may be made for a girl under the influence

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