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 1893) for large telescopes is shown in fig. 12. Quick motion in position-angle for rough setting or for the measurement of close double stars is given by the large ring R.

EB1911 - Micrometer - Fig 11.png . 11. The micrometer is clamped in position-angle by the screw K and slow motion in position-angle is given by the screw p. The small drum-head T opposite the micrometer head S turns a screw which acts upon a short cylinder that cannot turn but can move only in the direction of the axis of the micrometer screw. The end-plane of this cylinder receives the pressure of the micrometer screw, so that by turning the small drum-head the coincidence-reading of the movable web with the fixed web can be changed, and thus any given angle can be measured with different parts of the micrometer screw in order to eliminate the effects of periodic error of the screw.

EB1911 - Micrometer - Fig 12.png . 12.

The electric lamp a gives illumination of the webs in a dark field, nearly in the manner described for the Cape transit circle micrometer; the intensity of illumination is regulated by a carbon-resistance controlled by the screw b. The lamp c illuminates the drum-head and also, by reflection, the portions of the position-circle which come under the microscopes d and e. The head f is a switch which enables the observer to illuminate lamp a or c at pleasure. These lamps, although shown in the figure, are in reality covered so as not to shine upon the observer’s eye. The illumination of the field is given by a lamp near the object glass, controlled by a switch near the micrometer.

Repsolds in more recent micrometers under construction give a second motion to the eyepiece at right angles to the axis of the micrometer screw; this enables the observer to determine the zero of position-angle for his movable webs with the same accuracy as he formerly could only do for the so-called position-angle webs. Repsolds also provide two insulated sliding contact rings instead of the single ring g, so that the electric current for illuminating the lamps does not pass through the instrument itself but may come to the micrometer from the storage battery through two insulated leads. The same firm is also constructing a micrometer in which the readings of the head are printed on a band of paper instead of being read off at the time of observation.

Instruments have been invented by Alvan Clark and Sir Howard Grubb for measuring with the spider-line micrometer angles which are larger than the field of view of the eyepiece. In both cases two eyepieces are employed, one to view each separate web. One drawback to this form of instrument is that the two webs cannot be viewed simultaneously, and therefore the observer must rely on the steadiness of rate of the clockwork and uniformity in the conditions of refraction whilst the eye is moved from one eyepiece to the other.

Clark’s micrometer was exhibited at the June meeting of the Royal Astronomical Society in 1859 (Monthly Notices, R.A.S., vol. xix.). Grubb’s duplex micrometer is described in the 9th edition of the Encyclopaedia Britannica. Some examples of use of the latter are given by Professor Pritchard (Mem. R.A.S. xlvii. 4–12), who estimates the accuracy attainable with the duplex micrometer as equal to that of the heliometer; but as few measures of permanent value have been made with the instrument, and those made exhibit an accuracy far inferior to that of the heliometer, it is unnecessary to describe the instrument here in greater detail.

The Reading Micrometer-Microscope.—Micrometers used for subdividing the spaces on graduated circles and scales have, in general, only a single pair of cross-webs or parallel webs moved by a single screw. The normal form of the apparatus is shown in figs. 13 and 14. C is the objective, D the micrometer box, E the graduated head of the screw, G the milled head by which the screw cc is turned, A an eyepiece sliding in a tube B, aa (fig. 14) the slide, and b, b the spiral springs.

The focal length of the objective and the distance between the optical centre of the lens and the webs are so arranged that images of the divisions are formed in the plane of the webs, and the pitch of the screw is such that one division of the scale corresponds with some whole number of revolutions of the screw.

There is what is technically called a “comb” inserted in the micrometer box at d (fig. 14)—its upper surface being nearly in the plane of the wires. This comb does not move with reference to the box, and serves to indicate the whole revolution of which a fraction is read on the head. In fig. 14 a division is represented bisected by cross webs, and five revolutions of the screw correspond with one division of the scale. In all modern reading micrometers the cross webs of fig. 14 are replaced by parallel webs embracing the division

(fig. 15). The means for changing the length of the tube and the distance of C from the scale are omitted in the figure. These appliances are required if the "run" has to be accurately adjusted. By “run” is meant the difference between the intended whole number of screw revolutions and the actual measures of the space between two adjacent divisions of the scale in turns of the screw divided by the number of intended revolutions. In delicate researches two divisions of the scale should always be read, not merely for increased accuracy but to obtain the corrections for “run” from the observations themselves.

Repsolds employ for the micrometers of their reading microscopes the form of construction shown in fig. 9, omitting, of course, the motion of the whole micrometer box given by the screw s for those cases in which the axis of the micrometer is supposed to remain constant in position, as, for example, in the case of the reading microscopes of transit circles (see ).

But when the relative positions of two adjacent objects or scale divisions have to be determined (as, for example, in the case of heliometer scales), much time is saved by retaining the motion of the micrometer box. One double web, fixed in the box, is pointed symmetrically, as in fig. 15, on one of the scales, by moving the whole micrometer box by means of the screw s; the pair of webs, moved by the screw S, is then pointed upon an adjacent division on the other scale. If the reading for coincidence of the movable with the fixed webs is known, we then obtain from the single reading of S the difference from coincidence of the divisions of the two scales.