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Adjustments of the Reading Microscope.-Screw the objectglass home. Insert the body of the microscope into the collar destined to receive it, and screw home the nuts, nn and n'n'. Make the diaphragm and spider's lines visible distinctly, by putting the eye-piece, e, the proper depth into the cell, c. Then make the graduated limb also distinctly visible without parallax by turning the nuts, nn, and n'n', unscrewing one and screwing up the other till the desired object is attained.

Now bring the point of intersection of the spider's lines upon a stroke of the limb, and turn the micrometer head, m, to zero; then, turning the screw through five revolutions, if the point of intersection of the spider's lines has not moved over the whole of one of the divided spaces on the limb, the object lens must be screwed up to diminish the power by turning the cone, dd; and if it has moved over more than one of the divided spaces, it must be unscrewed to increase the power, and then altering the position of the microscope, by turning the nuts, nn and n'n', till distinct vision of the limb is again obtained, the measure of the space, moved over by five revolutions of the screw, must be repeated, as before When, after repeated trials, the result is satisfactory, the three nuts, nn, n'n', and b b, must be screwed tight home, to render the adjustment permanent.

When the microscope has been thus adjusted for distance the zero of the division on the limb must be brought to the point of intersection of the spider's lines, and the divided head, m, turned, till its zero is pointed to by its index, and then, if the zero on the comb, cc, be not covered exactly by the index, i, the comb must be moved by turning the screw, i, which enters the bottom of the micrometer box, till its zero is covered by the index pin. The adjustment of the reading microscope will now be perfect; and the graduated limb to be read by it, being divided at every five minutes, the degree and nearest five minutes of an observed angle will be shown by the pointer or index to this graduated limb; while the number of complete revolutions, and the parts of a revolution, of the screw. in the order of the numbers upon the micrometer head, m, required to bring the point of intersection of the spider's lines upon a division of the graduated limb, will be the number of minutes and seconds, respectively, to be added to the degrees and minutes shown by the index of the circle. The complete revolutions, or minutes, to be added, are shown by the number of teeth the index, i, has passed over from zero, and the parts of a revolution, or seconds and

tenths to be added, are pointed out upon the micrometer head m, by its index.

THE COLLIMATOR.

B B, is a rectangular mahogany box partly filled with mercury. F F, is a float of cast iron partly immersed in the mercury. bb, are two iron-bearing pieces, screwed to the bottom of the box by short iron screws; and each of these pieces has two vertical plates turned up, the inner one of which has a longitudinal slit in it, into which slits iron pivots, screwed into the sides of the float, are admitted The use of these parts is to keep the sides of the float parallel to the sides of the box, and at an inch, or more, from contact with any part of the box, that the mercury may assume a flat surface.

H and

K are two holding pieces of metal cast along with the float, and are perforated, to receive each a socket. The socket at H receives an achromatic object-glass, and is adjustable by a screw for its focal distance, and the socket at к holds two cross wires; while another socket, let into the end of the box at L, carries a lens forming an eye-piece; so that the collimator is in fact an astronomical telescope with a system of cross wires in the common focus of the object-glass and eye-lens. The inclination, as com

B

B

M

pared with the surface of the fluid, of the optical axis of this telescope, or of the line joining the center of the object-glass and the intersection of the cross wires, can be modified by the addition of perforated pieces of iron, held steady by the vertical pin, P, and by their weight depressing the end of the float. The mercury must be as pure as can be obtained, and particles of dust must be constantly excluded by a lid that covers over the top of the box. At м is a circular hole, closed when the instrument is not in use, through which the telescope, of which the error of collimation is sought, is to be directed; and a lamp is placed behind the eye-lens at L, to illuminate the cross wires.

Use of the Collimator with an Altitude and Azimuth Instrument.-Place the collimator in the plane of the meridian on the south side of the observatory, and direct it so that the cross wires of the telescope of the altitude and azimuth instru

ment may be seen through it, in the center of the field of view; then also will the cross wires of the collimator be seen. through the telescope in the center of its field of view. Read off the altitude of the cross wires of the collimator, and then, turning the instrument half round in azimuth, observe again the cross wires of the collimator, and read off the angle upon the vertical limb, which will now be a zenith distance. The difference between the sum of these readings and 90°, is the correction which is to be applied to the altitudes and zenith distances observed with the instrument.

Example.-The sun's meridian altitude had been observed on the 20th December, 1826, and the following determination of the error was made immediately after the observations were finished; viz:

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The collimator may also be used for a meridian mark with the transit instrument. When used with a circle for measuring altitudes and zenith distances, which has no motion in azimuth, the collimator must be moved from the north to the south side of the observatory, and the mean of the observations in each of these two positions will give the correction for the errors of collimation, &c., as above.

PART V.-ON THE GONIOMETER.

THE last instrument to which we shall call attention in this little work, is Wollaston's Goniometer, used for measuring the angles of crystals. The following lucid description of the construction and method of using this instrument is extracted from the able article on Crystallography in the "Encyclopædia Metropolitana

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The instruments used for measuring the angles at which the planes of crystals incline to each other, are called Goniometers.

"The mutual inclination of any two planes, as of a and b, fig. 1, is indicated by the angle formed by two lines, ed, ef, drawn upon them from any point, e, on the edge at which they meet, and perpendicular to that edge.

"Now it is known that if two right lines, as gf, dh, fig. 2, cross each other at any point, e, the opposite angles, def, geh, are equal. If, therefore, the lines, gf, dh, are supposed to be very thin and narrow plates, and to be attached together by a pin at e, serving as an axis to permit the point, f, to be brought nearer either to d, or to h, and that the edges, ed, ef, of those plates, are applied to the planes of the crystal, fig. 1, so as to rest upon the lines, ed, ef, it is obvious that the angle, geh, of the moveable plates would be exactly equal to the angle, def, of the crystal.

Fig. 1.

d

Fig. 2.

a

"The common goniometer is a small instrument for measuring this angle, geh, of the moveable plates. It consists of a semicircle, fig. 3, divided into 360 equal parts, or half degrees, and a pair of moveable arms, dh, gf, fig. 4, the semicircle having a pin at i, which fits into a hole in the moveable arms

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"The method of using this mstrument is to apply the edges, de, ef, of the moveable arms to the two adjacent planes of any crystal, so that they shall actually touch or rest upon those planes in directions perpendicular to their edge. The arm, dh, is then to be laid on the plate, m n, of the semicircle, fig. 3, the hole at e being suffered to drop on the pin at i, and the edge nearest to h of the arm will then indicate on the semicircle, as in fig. 5, the number of degrees which the measured angle contains.

"When this instrument is applied to the planes of a crystal, the points, d and f, fig. 4, should be previously brought sufficiently near together for the edges, de, ef, to form a more acute angle than that about to be measured. The edges being then gently pressed upon the crystal, the points, d and f, will be gradually separated, until the edges coincide so accurately with the planes that no light can be perceived between them.

Fig. 5

"The common goniometer is, however, incapable of affording very precise results, owing to the occasional imperfection of the planes of crystals, their frequent minuteness, and the difficulty of applying the instrument with the requisite degree of precision.

"The more perfect instrument, and one of the highest value to crystallography, is the reflecting goniometer, invented by Dr. Wollaston, which will give the inclination of planes whose area is less than 1000 of an inch, to less than a minute of a degree.

"This instrument has been less resorted to than might, from its importance to the science, have been expected, owing, perhaps, to an opinion of its use being attended with some difficulty. But the observance of simple rules will render its application easy.

"The principle of the instrument may be thus explained :

"Let ab, fig. 6, represent a

crystal, of which one plane only is visible in the figure, attached to a circle, graduated on its edge, and moveable on its axis at o; and let a and b mark the position of the two planes whose mutual inclination is required.

"And let the lines, oe, og, represent imaginary lines, resting on those planes in directions perpendicular to their common edge, and the dots at i and h, some permanent marks in a line with the center, o.

i.

130

Fig. 6.

19

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"Let the circle be in such a position that the line, o e, would pass through the dot at h, if extended in that direction, as in fig. 6.

"If the circle now be turned round with its attached crystal, as in fig. 7, until the imaginary line, og, is brought into the position of the line, o e, in fig. 6, the number 120 will stand opposite the dot at i. This is the number of degrees at which the planes a

and incline to each other. For if the line og be extended in the direction oi, as in fig. 7, it is obvious that the lines, o e, oi, which are perpendicular to the common edge of the planes, a and b, would intercept exactly 120° of the circle. "Hence an instrument constructed upon the principle of these diagrams is capable of giving with accuracy the mutual inclination of any two planes which reflect objects with sufficient distinctness, if the means can be found for placing

120

150

06

Fig. 7.

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them successively in the relative positions shown in the two preceding figures.

"This purpose is effected by causing an object, as the line at m, fig. 8, to be reflected successively from the two planes, a and b, at the same angle. It is well known that the images of objects are reflected from bright planes at the same angle as that at which their rays fall on those planes; and that

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