After all, the use of the object•glass micrometer is attended with difficulties, arising from the alterations in the focus of the eye, which are apt to cause it to give different measures of the same an gle at different times. To obviate these difficulties, Dr. Maskelyne, in 1776, con trived a prismatic micrometer, or a mi crometer consisting of two achromatic prisms, or wedges, applied between the object-glass of an achroma tic telescope, by moving of which wedges nearer to or further from the object-glass, the two images of an object produced by them appeared to approach to, or recede from, each other, so that the focal length of the object-glass becomes a scale for measuring the angular distance of the two images. The rationale and use of this micrometer are explained in the Philos. Trans. vol. lxvii.
Mr. Ramsden has described two new micrometers, which he has contrived for remedying the defects of the object-glass micrometer. One of these is a catoptric micrometer, which, besides the advan tage it derives from the principle of re flection, of not being disturbed by the heterogeneity of light, avoids every de fect of other micrometers, and can have no aberration, nor any defect arising from the imperfection of materials, or of exe cution, as the great simplicity of its con struction reqtures no additional mirrors or glasses;to those required for the tele scope; and the separation of the image being effected by the inclination of the two specula, and not depending on the focus of lens or mirror, any alteration in the eye of an observer cannot affect the angle measured. It has peculiar to itself the advantages of an adjustment, to make the images coincide in a direction per pendicular to that of their motion ; and also of measuring the diameter of a planet on both sides of the zero; which will ap pear no inconsiderable advantage to ob servers, who know how much easier it is to ascertain the contact of the external edges of two images than their perfect coincidence.
The other micrometer, invented and described by Mr. Ramsden, is suited to the principle of refraction. This micro meter is applied to the erect eye-tube of a refracting telescope, and is placed in the conjugate focus of the first eye-glass;. as the image is considerably magnified before it comes to the micrometer, any imperfection in its glass will be magnified only by the remaining eyeglasses, which in any telescope seldom exceeds five or six times ; and besides, the size of the micrometer glass will not be the 100th part of the area which would be required, if it were placed at the object-glass; and yet the same extent of scale is preserved, and the images are uniformly bright in every part of the field of the telescope. See Philos. Trans vol. lxix.
In the Philos. Trans. for the year 1782, Dr. Herschel, after explaining and imperfections of the parallel-wire mi crometer, especially for measuring the apparent diameter of stars, and the dis tances between double and multiple stars, describes one for these purposes, which he calls a lamp micrometer; one that is free from such defects, and has the ad vantage of a very enlarged scale. la speaking of the application of this instru ment, he says, " It is well known to op ticians, and others, who have been in the habit of using optical instruments, that we can with one eye look into a microscope, or telescope, and see an object much magnified, while the naked eye may see a scale upon which the magnified picture is thrown. In this manner I have gene rally determined the power of my tele. scopes ; and any one, who has acquired a facility of taking such observations, will very seldom mistake so much as one in fifty in determining the power of an in strument, and that degree of exactness is fUlly sufficient for the purpose.
"The Newtonian form is admirably adapted to the use of this micrometer, for the observer stands always erect, and looks in a horizontal direction, notwith standing the telescope should be elevated tu the zenith. The scale of the microme ter; at the convenient distance of 10 feet from the eye, with the power of 460, is above a quarter of an inch to a second ; and by putting on my power of 932,1 ob tain a scale of more than half an inch to a second, without increasing the distance of the micrometer ; whereas the most perfect of my former micrometers, with the same instrument, had a scale of less than the 2000th part of an inch to a se cond.
"The measures of this micrometer are not confined to double stars only, but may be applied to any other objects that require the utmost accuracy, such as the diameters of the planets or their satellites, the mountains of the moon, the diameters of the fixed stars, &c." We shall now give an account of a mi crometer by Mr. Troughton, which is ap plied to the eye-piece of a telescope to measure exceedingly small angles, as the diameter of the heavenly bodies, &c. Plate Microscope, &c. fig. 6, is an ortho graphy projecting endways; fig. 7, a sec tion of the box containing the wires; and fig. 8, a section lengthways; the same letters, as far as they can, are used in all the figures. Figs. 6 and 8, A is an eye tube containing a convex lens at each end, this slides in another tube, d d, so as to adjust the glass to distinct vision of the wires : the tube, d d, is screwed into ano ther, 64, which is much larger; through this a thin long box, D D D, containing the wire, slides. The micrometer is screw ed to the telescope by a male screw, e e, (fig. 8.) in the same piece with which is a circularplate, ff, cut all round with fine teeth; this plate fits against the flat bot tom of the box, 6, and turns round con centrically with it by means of a ring, k, fitting into a conical hole in the centre of the plate/f, and screwed to the box ; a small endless screw, A, (fig. 6,) turning in two brass collars screwed to the box, 6, works in the teeth cut round the plate, f, and by that means, when the milled head on the arbor of the endless screw is turn ed, it turns the eye-tube and box, D D, round, to bring it to any convenient peal tiokfor measuring the angles required ; containing the wires is shewn open in fig. 7, it containing two frames, it A h, and 1 11 1, one sliding within ano ther, which moves in the box without lateral shake, yet fitted so as to slide easi ly backwards and forwards in the box, by the screws, m and n, in the same manner as the microscope in the upper part of the same plate ; o and p are springs to counteract the screws and make the mo tion pleasant. A wire is stretched across the frame, A h, at right angles to its sides, and another of the same size is fixed a cross the slider, 111 4 exactly parallel to the former; a small quantity of the un der side of the latter is cut away, and its wire is fixed to another plane to the wire of h hh, so that the wires can pus each other without touching, but as near as possible ; when they are placed by their screws over each other, and viewed through the eye-tube, they appear but as one wire : the divided circle x, on the nuts of the screws are then slipped round, without the screw, to bring the first divi sion on them to the index, 1; the instru ment is now adjusted for observing any angle ; it is screwed to the telescope, and by the endles screw A, (fig. 6.) the mi crometer is turned round so as to bring a fixed wire, w, which is perpendicular to the others, to cover the two objects; the two wires are then separated by turn ing either of the nuts, F, until the wires include the angle to be measured: the whole box (fig. 7.) of the micrometer slides through the tube, in the direction of its length, to follow any moving object. When the observation is completed, it is read off by a scale of notches in the box, (fig. 7.) determining the number of re volutions the screw has made, and the di visions pointed out on the circles, x ; by the indexes, 11, the number of aliquot parts is denoted ; the circular plate, ff, is divided into degrees, as shewn in fig. 6, and it is by this that the angle the line measured makes with the horizon is re gistered.