250 MICROMETER [DOUBLE-IMAGE Fig. 27. he mentions "because the glasses in these two sorts are somewhat prismatieal, but mostly those of the first model, which could therefore bear no great charge " (magnifying power). A third model proposed by Savary consists of two complete lenses of equal focal length, mounted in cylinders a side by side, and attached to a strong brass plate (fig. 26). Here, in order to fulfil the purposes of the pre vious models, the distance of the centres of the lenses from each other should only slightly exceed the tan- ent of sun s diameter x focal length of lenses, avary dwells on the difficulty both of procuring lenses sufficiently equal in focus and of accurately adjusting and centring them. Bouguer. In the Mem. Acad. de Paris, 1748, Bouguer de scribes an instrument which he calls a heliometer. Lalande in his Astronomie (vol. ii. p. 639) mentions such a heliometer which had been in his possession from the year 1753, and of which he gives a representation on Plate XXVIII., fig. 186, of the same volume. Bouguer s helio meter was in fact similar to that of Savary s third model, with the important difference that, instead of both object-glasses being fixed, one of them is movable by a screw provided with a divided head. No auxili ary filar micrometer was required, as in Savary s heliometer, to measure the interval between the limbs of two adjacent images of the sun, it being only necessary to turn the screw with the divided head to change the distance between the object-glasses till the two images of the sun are in contact as in fig. 27. The differences of the readings of the screw, when converted into arc, afford the means of measuring the variations of the sun s apparent diameter. Dollond. On the 4th April 1754 Dollond com municated a paper to the Royal Society of London (Phil. Traits., vol. xlviii. p. 551) in which he shows that a micrometer can be much more easily constructed by dividing a single object-glass through its axis than by the employment of two object-glasses. He points out (1) that a telescope with an object- glass so divided still produces a single image of any object to which it may be directed, provided that the optical centres of the segments are in coin cidence (i.e., provided the segments retain the same relative positions to each other as bafore the glass was cut) ; (2) that if the segments are separated in any direction two images of the object viewed will be produced ; (3) that the most convenient direction of separation for micrometric purposes is to slide these straight edges one along the other as the figure on the margin (fig. 28) represents them : " for thus they may be moved without suffering any false light to come in between them ; and by this way of removing them the distance between their centres may be very conveniently measured, viz., by having a vernier s division fixed to the brass work that holds one segment, so as to slide along a scale on the plate to which the other part of the glass is fitted." Dollond then points out three dif ferent types in which a glass so divided and mounted may be used as a micrometer : "1. It may be fixed at the end of a tube, of a suitable length to its focal distance, as an object-glass, the other end of the tube having an eye-glass fitted as usual in astronomi cal telescopes. " 2. It may be applied to the end of a tube much shorter than its focal distance, by having another convex glass within the tube, to shorten the focal distance of that which is cut in two. "3. It may be applied to the open end of a reflecting telescope, either of the Newtonian or the Cassegrain construction." Dollond adds his opinion that the third type is " much the best and most convenient of the three" ; yet it is the first type that has survived the test of time and experience, and which is in fact the modern heliometer Fig. 29 illustrates Dollond s divided object-glass heliometer of the third type. A is the end of the reflecting telescope, upon which the adapter B is fitted. B carries a wheel (not seen in the figure) formed of a ring racked at the outer edge, and fixed to the brass plate CC, so that a pinion moved by the handle D may turn it into any position. Two plates F, G, Pi* Fig. 29. Fig. 30. with the attached semi-lenses, move in slides fixed to the plate CC, simultaneous motion, in contrary directions, being communicated to them by turning the handle E, which drives a concealed pinion that works in the two racks seen in the highest part of the figure. The amount of separation of the semi-lenses is measured by a scale 5 inches long, subdivided to ^Vth of an inch, and read by a vernier on the plate F to -^^th of an inch. In practical use this micrometer has never given satisfactory results (see Mosotti in the Efemeride of Milan for 1821). It must be remembered, however, that when Dollond gave preference to this type he had not invented the achromatic object-glass ; his preference was fully justified under these circumstances. So far as we know no heliometer with a divided achromatic object-glass was ever made by the elder Dollond on the principle of his first type. His son, however, made what he called an object-glass micrometer, which was a great improvement on the elder Dollond s second type. In the older construction the brass mountings of the semi-lenses obstructed the light entering the telescope in proportion to their separation, and the images were so coloured as to prevent the use of any but very low powers. In the later construction the movable segments are formed from a negative achromatic lens of much larger aperture than the object-glass of the telescope with which the micrometer is employed ; and, for convenience in mounting, the segments gbh and cdf (fig. 22) are removed. In the fine example of this instrument at the Royal Observatory, Cape of Good Hope, the movable lenses consist of segments of the shape gach and cacf (fig. 22) cut from a complete negative achromatic combination of 8| inches aperture and about 41- feet focal length, composed of a double concave flint lens and a double convex crown. This is applied to an excellent achromatic telescope of 3 inches aperture and 42 inches focal length. The instrument is represented in fig. 30 ; the same letters indicate the analogous parts of fig. 29. The frame CC, moved by teeth on its outer edge, carries one of the halves G of the lens, and a simi lar frame with teeth car ries the other half F. A scale 8^ inches long is fastened like an edge-bar to the frame of the seg ment G, and each inch is subdivided into twenty parts, which are read off by a vernier to nnsirth of an inch, and, by estimation, this can easily be carried to -^^th or ^^^th of an inch. The two movable frames are imbedded" in a fixed plate HH, screwed to the adapter B, having a circular hole in its middle equal to the diameter of the object-glass. The slide of the segment G is moved by turning the milled head to the right of A, and the other segment F by means of a rack and pinion on the opposite side, the latter being turned from the eye-end by a handle not seen in the figure. A screw is provided for clamping the slide of the segment G, as it is intended that only the segment F shall be moved in making the final bisection. There is an index attached to the slide of G, reading on a rough scale engraved on the plate H, which is obviously intended for setting the optical centre of the segment G approxi mately as far from the optical axis of the telescope on one side as the optical centre of the segment F will be on the other side during the intended measurement. This arrangement not only permits the measurement of angles twice as great as would be possible if one segment were fixed, but is also important in increasing the symmetry of the measures. The vernier is placed at one end of the scale when the optical centres of the segments are in coincidence, and is provided with screws at I, which are intended for adjusting the zero of the scale. The younger Dollond has in this model retro graded, in some respects, from the admirable example of his father, who, as shown in fig. 29, not only gave the lenses automatic opposite motion symmetrically with respect to the axis of the telescope, but seems also to have provided for entire elimination of index error by making it possible to observe all angles on opposite sides of zero a precaution possible in the later form only when very small angles are measured. Rotation of the micrometer in position angle is provided for as in the earlier form, but the in strument is not furnished with a position circle. With one of these instruments of somewhat smaller dimensions Tries- ^ (telescope 2^ inches aperture and 3^ feet focus) Triesnecker made a necker s series of measurements at the observatory of Vienna which has measure been recently reduced by Dr Schur of Strasburg (Nova Ada der ments. Ksl. Leop. -Carol. Deutschcn Akademic dcr Natursforsclicr, xlv. No. 3). The angle between the stars and g Ursse maj. (708" 55) was measured on four nights ; the probable error of a measure on one night was 0" 44. Jupiter was measured on eleven nights in the months of June and July 1794 ; from these measures Schur derives the values 35" 39 and 37" 94 for the polar and equatorial diameter respectively, at mean distance, corresponding with a compression
1/1 4 44. These agree satisfactorily with the corresponding values