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ART. XLII. An account of an Indicator Stage for Microscopes; by A. S. JOHNSON, Albany, N. Y.

IN October last Messrs. Grunow of New Haven completed for me and adapted to my microscope a moveable stage, upon the principles stated by Prof. Bailey in his article upon the Universal Indicator. Carrying out his suggestions, they have succeeded in overcoming the difficulties which were pointed out as likely to attend attempts to apply the indicator to moveable stages. The indicator stage combines all the convenience of the best moveable stages, with all the advantages to its possessor of the universal indicator. By means of it all parts of a slide can be rapidly brought by equal sweeps under examination and the noteworthy objects registered. The registration is identical with that obtained by the universal indicator.

The stage is composed, as is usual, of three plates of which the lower is fixed. Upon it the other two move 13th inches from side to side; the top plate moves forward and back 14th inches upon the middle plate. The motions are effected by racks and pinions, the milled heads of which are under the stage. In the bottom plate are inlaid two scales graduated in 50ths of an inch, which extend from the middle of the side edges towards the centre of the stage. That on the left is graduated from 0 to 50; that on the right from 60 to 110. In the middle plate are two similar scales, extending from the middle of the front and rear edges of the plate towards its centre. The front scale is graduated from 20 to 50, the other from 50 to 80. Over each scale projects a steel index attached to the plate above, to facilitate accurate reading of the scales. Upon the top plate is engraved a horizontal line passing across the stage, intercepted of course by the hole in the centre, but so drawn that continued across the hole it bisects horizontally the field of view. At right angles to this line two others are engraved each crossing it on opposite sides of the centre at the distance of one inch from the centre of the field of view, which is also the centre of the stage.

The position of the several scales is adjusted with reference to these lines as follows. When the horizontal line, continued across the hole in the stage, bisects the field of view, the indexes attached to the front and rear edges of the top plate stand at 50 on the two scales of the middle plate. As upon the universal indicator the horizontal line at 50 also passes through the centre of the field of view, it is obvious that the positions of that line and of the horizontal line on the stage are identical.

The verticals being each an inch distant from the centre of the field of view at their intersection with the horizontal line, and one inch being also the distance on the universal indicator

from its centre to the points 40 and 70 in its horizontal line, the right hand index is made to stand at 70 on the right scale and the left hand index at 40 on the left scale. When the stage is set at 70 and a slide placed on the stage with its right vertical in coincidence with the right vertical on the stage, its position. will be the same as if the slide were placed upon the universal indicator, with the right vertical at 70 of its horizontal graduation. It is hardly necessary to add that a motion of the stage carrying the slide is exactly equivalent to a motion in the same direction and of the same extent of the slide alone upon the universal indicator.

The top plate of the stage is furnished with a slide holder moving in grooves, by means of which a slide may be easily adjusted so that its horizontal guide line shall coincide with the horizontal line on the stage. This adjustment having been made, its right or left vertical is to be made to coincide with the corresponding vertical on the stage. The right verticals are to be used for numbers of the horizontal graduation from 60 to 110, and the left for those from 0 to 50.

In case of an object already registered, all that remains to be done is to move the stage by the milled heads of the pinions till the indexes stand at the registered numbers, and the object will at once be found. In case of a slide not registered when an object is found which it is desired to record, a glance at the indexes will give the proper numbers to be recorded.

It will be noticed that the vertical graduations extend only from 80 to 20. This being equal to 13th inches, affords ample room for all ordinary slides not covered with paper. But as some slides are still so covered, provision has been made for them. On the stage parallel to the horizontal line two others are engraved each ths of an inch distant from the first. This distance being exactly equal to 20 divisions on the scales, in order to find an object registered by the universal indicator from the front edge of the slide, the same edge must be brought to the front line and the stage be set at 20 more than the registered number. If on the other hand the registration has been made by the lower edge, the rear line must be used and the stage set at 20 less than the registered number. In registering objects on such slides by the indicator stage, the registration ought always to be made as the numbers would appear on the universal indicator. To effect this it is only necessary, if the front line be used, to deduct 20 from the number indicated, if the rear line to add 20. The manipulation with this stage is by no means complicated. With the stage before you, it can be mastered in ten minutes, and when once mastered its convenience and value are great. Registered objects can be found and new ones registered with great rapidity. To illustrate the great saving of time which it effects,

I may say, that in less than four minutes, I put upon the stage and adjusted a slide and then found and brought to the centre of the field of a 1-inch object-glass ten recorded objects, for each of which both sets of numbers were different. It is true that I made haste, but I know no other method by which it could have been done in twice the time. One minute may then be reckoned as the maximum time requisite to find any recorded object. How great a gain this is, every one will feel, whose time and patience have both been many times exhausted in searching to rediscover some minute object, before the indicator was invented.

ART. XLIII. On the Earthquake of April 2, 1851, in Chile; by Lieut. J. M. GILLISS, A.M., U. S. N.*

FOR several days previous to April 2nd, the sky had been unusually overcast, the barometer fluctuating as it does during winter rain-storms. Not far from 9 o'clock, on the the night of the 1st, there was a vivid, quick flash of lightning to the NNE, so intense in brightness as to illuminate within the observatory, where I had been at work some hours. I was startled by the sudden brilliancy, and listened for close-following thunder, but no sound came; neither was the flash repeated, nor was there the smallest speck of cloud even about the horizon in that direction.† Coming down the hill, about midnight, my left eye was found to be injured by over-exertion; and the pain which soon followed brought on nervous restlessness that kept me awake several hours. Sleep, long courted, came so profoundly at last, that when nature, in wrath, was shaking the city on its foundations, and a startled population fled with cries of terror, though roused by the incipient shock, nearly half its violence had passed before full consciousness returned. Habit brought me instantly to the floor, watch in hand, and in such a position that I could embrace, at a glance, the roofs across the street, a little mirror directly in front, and the wash-stand diagonally to the right. But reason was torpid. Though there was a consciousness of excessive oscillation of the floor, and most infernal subterranean roarings; a recognition that the pictures of the paper on the opposite wall were waving from side to side across the mirror; a conviction

From the Report of the U. S. Naval Astronomical Expedition to Chile, by Lieut. Gilliss, vol. i, p. 108.

Many of the most intelligent persons in Chile regard earthquakes as due wholly to electrical agency; and as we have no right to reject popular belief until every phase of the phenomenon is satisfactorily explicable without such influence, it is proper that the occurrence of such remarkable lightning-so short a time before the shock, and in the direction from which it came-should not be omitted. For the same reason Humboldt mentions (Relation Historique, Liv. IV, Chap. 10) "two strong shocks simultaneously with a clap of thunder."

that the roofs and tiles of the houses in front were "dancing like mad;" and knowledge that the affrighted people were invoking the mercy of their God in the utmost distress, and the rattle of windows and doors was making no small addition to the uproar, still several seconds must have elapsed before I could realize the actual magnitude of the storm agitating the crust of our abidingplace. Nevertheless, experience having taught that the phenomenon is of little continuance, there was sufficient rationality to prevent my leaving the room; and I stood with senses gradually returning, thinking each vibration would be the last. But I watched and watched the dial of the monitor in my hand, and, instead of subsiding, there came accessions to the force of the moving power with each beat of its balance-wheel, till the walls. on either side were swaying to and fro, the plank ceiling screeching overhead, and finally the doors flew open, exhibiting the opposite room filled with a cloud of dust, and its floor covered with broken adobes, which had fallen between the ceiling and walls. Half a minute had now elapsed, each second of which seemed at least a day; and in the fiercest violence, as the creaking of the ceiling was too ominous to disregard longer, I found myself creeping for shelter beneath the lintel of the door. Of a sudden the wall swayed away from the roof, showing the blue sky above, and a mass of rubbish fell, blinding and almost stifling me; so that it became necessary to take refuge under the lintel of the outside door, where fresh air might be obtained. As the tiles. were falling in a shower from the roofs, escape to the patio was more hazardous than to remain under the doorway; for one had better risk being partially buried than have his head split with one of these heavy pieces of earthenware.

The motion had now become fearful, and the roar of the pent up vapor, as it moved heavily along, most awful; yet every little while there would reach me the clear ringing laugh of one of the assistants-inspired by the efforts of a companion to attain a place of greater apparent safety-marked contrasts of expressed human sensations in this terrestrial convulsion. I was not conscious of fear at any instant, nor was it possible to make the mind realize that the house might fall, although the walls were breaking all round, and at every few seconds the sky was visible through their crevices; but there was a sensation of dread-a feeling of absolute insignificance in the presence of a power that shook the Andes as willows in the breeze. I was humbled to the dust. Afterwards I learned, that among the mass in the streets there was but one thought, one desire-flight. But where fly to? The massive stone arches of the sanctuary had been broken, their key-stones had partially fallen, and the priests had been driven from the altars by masses of masonry precipitated around them; the hills were shaking huge rocks from crests

where they had slumbered since the dawn of nature, their trains marked by streams of fire; in the streets tiles were falling in showers mid clouds of dust; and on the open plain, in addition to that most unearthly and distressing noise and the moaning of cattle in their brute terror, the trees were waving from side to side under the influence of that same unseen but omnipotent agent.

North

Preceded some seconds by the usual rumbling noise, the first shock commenced at 6h 48m 10s A. M., and for eighteen seconds continued with nearly uniform violence, equal to, and in the kind of motion not altogether unlike, that of December 6th. This started the tiles and walls, though it broke nothing, a fact which may perhaps be accounted for by the greater rapidity with which the atoms at the surface of the earth were disturbed on the last occasion; for, if one may judge from bodily sensations, the shaking was certainly as great as in December, though the effects. were much less. The most excessive displacements were between 6h 48m 288 and 6h 48m 53s; and at 6h 49m 38s terminated an earthquake unparalleled in central Chile since 1822. Less than one minute and a half: a brief period of one's life when marked only by events of ordinary occurrence-but an age when one stands on a world convulsed. Beginning at 6h 48m 28s, the oscillations, then quite distinct, rapid, and abrupt, were of such magnitude that one involuntarily sought support; and though this actually lasted only twenty-five seconds, the time seemed endless, when measured by the multitude of thoughts crowded into it. Liquids were tossed to the north and south; and at the end, the surface of the mercury in a cup with vertical sides was left 14 inch below the rim. A barometer suspended on a north and south wall was thrown down, and all objects not summarily shaken off were moved by successive jolts to the north or south; generally in the latter direction. Every wall in the house was broken, some of them so that day-light shone through; others were thrown permanently out of the vertical, and scarcely a tile remained in place on the roof. Our pendulum was still gyrating when we could venture into the room where it was kept. Unfortunately the board supporting

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