to the object-glass. It consists of an eye-piece, furnished with a revolving diaphragm, containing apertures of different sizes, each of which can be brought into the centre of the field of view. By sliding one of these apertures towards or from the object-glass, the cone of rays is respectively reduced or augmented, which has the same effect as diminishing or increasing the aperture of the object-glass. A scale affixed shews the amount of the diminution of the aperture.

Objects near the zenith are frequently inconvenient to observe; to meet this difficulty a diagonal eye-piece is employed; this is merely a bent tube containing a small reflector, inclined at an angle of 45°. The reflector may be either of polished speculummetal, or a glass, or rock-crystal prism. The prism is to be pre

Fig. 193.

H

ferred, as much less light is lost in the transmission, and the eye-piece thus arranged becomes serviceable for observing the Sun. This luminary is better viewed by reflection than directly, for several reasons: amongst others, the loss of light is no disadvantage, and by using a prism the rays of heat, always very inconvenient, almost entirely pass away into the air without traversing the magnifier, and endangering the glasses and the observer's eye. With apertures greater than 2 inches, direct vision of the Sun becomes very hazardous. A special eye-piece for the Sun has been devised by Mr. Dawes e, and works well, but it is rather expensive. It consists of a revolving circular metallic plate, faced on the inner side with ivory, containing a series of apertures of various sizes from to an inch; these serve to limit the field ad libitum, and the field so curtailed is examined by single lenses mounted on another plate revolving concentrically with the former one. Superposed upon the wheel of single lenses is another

THE DIAGONAL EYE-PIECE.

• Month. Not. R.A.S., vol. xii. p. 167.

wheel, containing a series of dark glasses of various shades, to suit the eye and magnifying power used. The single lenses are focussed on the apertures in the diaphragm by a rack and pinion movement, and this renders the eye-piece as a whole complicated and expensive, though admirably adapted for solar observation. The wheel of dark glasses is made to slip off the eye-piece, so that if it is required it can be devoted to other uses. A more simple form of solar eye-piece is that which consists of an adapter in which a diaphragm plate is fitted as above. The wheel of lenses is replaced by a tube, into which fit the diminutive positive eyepieces required for use with a micrometer. Across the top of each eye-piece is a groove with cheeks, to retain in position a wedge of dark glass, capable of a sliding motion in front of the eye lens, for varying the intensity of the shade. The largest hole in the diaphragm plate should be equal to the field of the lowest power, so that the whole apparatus may, if desired, be advantageously used for other than solar purposes. It is a great feature in this construction that the rack and pinion adjustment for focussing is dispensed with. This is possible because all the positive eye-pieces have the same focus.

Further remarks on eye-pieces will be made in a subsequent chapter devoted to practical hints on observing. It may suffice to add here that a telescope once focussed will generally suit several observers of average sight; for near-sighted persons the eye-piece must be pushed in; for far-sighted persons, drawn out.

Fig. 194.

A micrometer is used for measuring small celestial distances. The simplest form is that known as the Reticulateds Micrometer. It consists of an eye-piece of low power, having stretched across it a number of wires at right angles to and at equal and known distances from each other. All that the observer has to do is to apply the eye-piece to the telescope, illuminate the field with the small lantern (if necessary, as it usually is at night), and notice how many divisions cover the

THE RETICULATED
MICROMETER.

f

1 μικρός sm all, and μέτρον a measure.

g Rete, a net.

object whose size it is desired to measure. Knowing the value of each division, the solution then becomes a simple matter of arithmetic. To determine the value of the divisions, turn the telescope on to a star at or very near the equator, note by a sidereal clock the time in seconds occupied by the star in crossing any convenient number of divisions. Multiply this by the cosine of the star's declination, and the product will be the interval in equatorial seconds; this multiplied by 15 will be the space in seconds of arc. Then divide this by the number of the divisions, and the quotient will be the arcual value corresponding to each.

This micrometer is at best but a crude contrivance: the more precise instrument is the parallel-wire micrometer. This in its elementary form may be thus described:-Two spider lines or wires are so mounted parallel on sliding frames that by suitable mechanism of screws &c. they can be made to coincide with each other, or to separate by a small distance. The revolutions of the screws serve to afford a measure of the angular distance travelled over when the angular value of each revolution is known..

Fig. 195.

THE PARALLEL-WIRE MICROMETER.

This apparatus is placed in the focus of the object-glass of the telescope so that the eye viewing the object, a measurement of which is desired, is enabled (by suitable illumination if necessary, as above) to see clearly the wires. Supposing a comet is visible, and the observer wishes to measure the diameter of its head, all he has to do is to bring the two wires together on one side of the comet, and then to turn one of the screws until the wire moved is brought to coincide with the other margin of the comet: the number of turns and parts of a turn necessary to effect this is a measure of the angular diameter of the object.

To determine the value of a revolution of the screw, separate the wires by any convenient number of revolutions, and turn the telescope on to an equatorial star: note the time in seconds occupied by the star in passing from one wire to the other; multiply this by the cosine of the star's declination, and the product will be the interval in equatorial seconds. This multiplied by 15 will be the space in seconds of arc. Then divide this by the number of the revolutions of the micrometer-screw, and the quotient will be the arcual value corresponding to each.

Another convenient method of determining this value is practised as follows:-Take a foot rule and place it at right angles to the optical axis of the telescope, at a distance, say, of 100 yards, and observe how many turns and parts of a turn are required to include its length. Ascertain by calculation the angle subtended by the rule, at the distance at which it is placed. Then if a is the angle subtended, and n the number of turns, v, the value of the same, will be represented by the following simple equation

a

n

which is merely the former rule given in another shape. In the case assumed of the foot rule, a is 11' 27.5"-a fact which it may save trouble to the reader to have stated in this place.

As constructed for use with a large telescope, the parallel-wire micrometer (the above description of which merely takes cognizance of fundamental principles) is a somewhat elaborate piece of apparatus. Spiral springs are inserted between the frames and the sides of the rectangular metal box, which protects the more delicate parts, for the purpose of assisting the screws in their work of driving inwards the frames carrying the wires. On one side of the field of view is a metal comb or notched scale of teeth, which correspond in size to the threads of the screw. Every fifth notch is cut deeper than the rest, and they are numbered from zero at the centre by tens in each direction. The zero is represented by a small circular hole, and every tenth notch has smaller circular holes drilled under it corresponding in number to the decades of

teeth above. The spider lines or wires coincide at zero. The screws have generally about 100 threads to the inch, and near their ends (which are milled-headed) are small circles graduated to 100 equal parts; it follows that the motion of the head through one of these divisions advances the wire through 100th of an inch. It will be readily understood, therefore, that with a micrometer thus constructed, angular distances of a very minute amount may be subjected to measurement 1.

The parallel-wire micrometer mounted so as to rotate at right angles to the optical axis of the telescope, and provided with a third and larger graduated circle concentric with the optical axis, becomes the position micrometer, used for measuring angles made with the meridian by lines joining double stars.

The method of observing such an angle may be thus briefly described. Make the line which is horizontal in figure 195 parallel

DIAGRAM ILLUSTRATING THE MEASUREMENT OF ANGLES OF POSITION.

to the equator by shifting it till the larger of the 2 stars passes along it during the whole of its passage across the field;

h For an account of a modification of this instrument by Alvan Clark for measuring large arcs, see Month. Not. R.A.S., vol. xix. p. 324.