THE READING MICROSCOPE.

The first of the annexed figures represents a longitudinal section of this instrument, and the second represents the

field of view, showing the magnified divisions of the limb of the instrument to which the microscope is applied, and the diaphragm, d d, of the microscope, with its comb, c c, and cross wires, w w The diaphragm is contained in the box, tt, and consists of two parts moving one over the other, the comb, c c, which is moved by the screw, i, at the bottom of the box, for the purpose of adjustment, and the cross wires, w w, and index, i, which are moved over the comb and the magnified image of the

limb, by turning the milled head, h. The micrometer head, m, is attached by friction to the screw turned by the milled head, so that, by holding fast the milled head, the micrometer head can be turned round for adjustment.

e is the eye-piece, which slides with friction into the cell, c, so as to produce distinct vision of the spider's lines of the micrometer. The object-glass, o, is held by a conical piece, d d, which screws further into or out of the body of the instrument, so as to produce distinct vision of the divided limb to be read by the microscope, and, when adjusted, is held firmly in its place by the nut, bb. The microscope screws into a collar, so as to be capable of adjustment with respect to its distance from the divided limb, and, when so adjusted, is held firmly in its place by the nuts, nn, n'n'.

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.

H and

B B, is a rectangular mahogany box partly filled with mercury. FF, 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 these pieces has two vertical plates turned up, the inuer of which has a longitudinal slit in it, into which slits iron pivois, 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 s : 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. K are two holding pieces of metal cast along with the float, and are perforated, to receive each a socket. The socket at н receives an achromatic object-glass, and is adjustable by a screw for its focal distance, and the socket at K 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:

Before reversion the apparent altitude of the cross

wires was

After reversion the apparent zenith distance of the cross wires was

0° 1' 2"-33

89 58 10 ·33

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 "Encyclo pædia Metropolitana."