Two or more sets of prisms may be adapted to the same telescope, to be used each in their turn, for the more commocious measurement of diff,:rent angles. Thus it mi) be very convenient to use one set of prisms for measuring angles not exceeding 36', and conse quently fit for measuring the diameters of the sun and moon, and the lucid parts and distances of the cusps in their eclipses : and another set of prisms to measure angles not much exceeding one minute, and conse quently fit for measuring the diameters of all the other planets. This latter set of prisms will be the more convenient for measuring small angles, on account of a small imperfection attending the use of this microme ter, as before mentioned ; namely, that angles cannot be measured with it when the prisms approach very near the focus of the object-glass, the pencils of rays being there lost at the point where the prisms touch one another." This micrometer has not been found to answer in practice. In addition to the ordinary uncorrected co lours of the object-glass, it is injured by the uncorrect ed colours of the achromatic prism, which most parti cularly affect the touching limbs.
A catoptric double image micrometer analogous to that of Dr. Maskelync, but much more simple in its construction, in consequence of no additional prisms, lenses, or mirrors, being used, has lately been proposed by Dr. Brewster. It is applied to the Newtonian te lescope, and has the advantage of not being affected by any change in the focal length of the eye An account of this micrometer will be found in the Edinburgh Philo sophical Journal for 1820.
In the year 1779, the late ingenious Mr. Ramsden communicated to the Royal Society an account of two micrometers ; a catoptric one, which formed two im ages by means of two semi-mirrors ; and a dioptric one, which formed a double image by means of two semi lenses placed in the eye-piece of the telescope The first of these instruments is represented in Plate CCCLXXV. Fig. 22, and has been thus described by its inventor. " Beside the advantage it derives from 'he principle of reflection, of not being disturbed by the hete rogeneity of light, it avoids every defect of other crotheters, and can have no aberration, nor any defect arising from the imperfection of materials or of execu tion; as the extreme simplicity of its construction re quires no additional mirrors or glasses to those required for the telescope ; and the separation of the images be ing effected by the inclination of the two specula, and not depending on the focus ol any lens or mirror, any alteration in the eye of an ooserver cannot affect tile angle measured. It has also the ol an ad
justment, to make the images coincide in a direction pet pendicular to that of their motion; and also of mea suring the diameter of a planet on both sides of the ze ro, which will appear no inconsiderable advantage to observers who know how much easier it is to asce•ain the contact of the external edges of two images than their perfect coincidence. In this micrometer, the small speculum A is divided into two equal parts ; one of which is fixed on the end of the arm B the other end of the arm is fixed on a steel axis X, which crosses the end of the telescope C. The other half of the mirror A is fixed on the arm D, which arm at the other end terminates in a socket y, that turns on the axis X ; both arms are prevented from bending by the braces a a. G represents a double screw, having one part c cut into double the number of threads in an inch to that of the part g; the part e having 100 threads in one inch, and the part g 50 only. The screw e works in a nut F in the side of the telescope, while the part g turns in a nut H, which is attached to the arm B ; the ends of the arms B and D, to which the mirrors are fixed, are se parated Irons each other by the point of the double screw pressing against the stud h, fixed to the arm D, and turning in the nut II on the arm B The two arms B and 1) are pressed against the direction of the double screw e g, by a spiral spring within the part n, by which means all shake or play in the nut H, on which the measure depends, is entirely prevented.
Front the difference of the threads on the screw at e and g, it is evident that the progressive motion of he screw through the nut will be half the distance of the centres of the two halves ol the mirror; and conse quently the half mirrors will be moved equally in con trary directions from the axis of the telescope C.
The wheel V fixed on the end of the double screw has its circumference divided into 100 equal parts, and the index I shows the motion of the screw with the wheel round its axis, while the number of revolutions of the screw is shown by the divisions on the same index. The steel screw at R may be turned by the key S, and serves to incline the small mirror at right angles to the direction of its motion. The telescope itself has a mo tion round its axis for the conyeniency of measuring the diameter of a planet in any direction ; and the inclina tion of the diameter measured with the horizon is shown by a level and veinier on a graduated circle, at the breech of the telescope.