IIELIOMErER.] MICROMETER 253 When Bessel ordered the Kcinigsberg heliometcr, he was anxious to have the segments made to move in cylindrical slides, of which the radius should be equal to the focal length of the object-glass. Fraunhofer, however, did not execute this wish, on the ground that the mechanical difficulties were too great. Wichmann states (KiJniysb. Bcobach., xxx. p. 4) that Bessel had indicated, by notes in his handbooks, the following points which should be kept in mind in the construction of future heliometers: (1) The segments should move in cylindrical slides ; l (2) the screw should be protected from dust; 2 (3) the zero of the position circle should not be so liable to change ; 3 (4) the distance of the optical centres of the segments should not change in different position angles or otherwise; 4 (5) the points of the micrometer screws should rest on ivory plates ; 5 (6) there should be an apparatus for changing the screen. 6 The elder Struve, in describing the Pulkowa heliometer, 7 made by Merz in 1839 on the model of Bessel s heliometer, submits the following suggestions for its improvement : 8 (1) to give automatic ally to the two segments simultaneous equal and opposite move ment; 9 and (2) to make the tube of brass instead of wood ; to attach the heliometer head firmly to this tube; to place the eye-piece perma nently in the axis of the telescope ; and to fix a strong cradle on the end of the declination axis, in which the tube, with the attached head and eye-piece, could rotate on its axis. Both suggestions are important. The first is originally the idea of Dollond (fig. 29) ; its advantages were overlooked by his son (description of fig. 30), and it seems to have been quite forgotten till resuggested by Struve. But the method is not available if the separation is to be measured by screws ; it is found, in that case, that the direction of the final motion of turning of the screw must always be such as to produce motion of the segment against gravity, otherwise the "loss of time" is apt to be variable. Thus the simple connexion of the two screws by cog-wheels to give them automatic opposite motion is not an available method unless the separation of the segments is independently measured by scales. Struve s second suggestion has been adopted in nearly all succeed ing heliometers. It permits complete rotation of the tube and measurement of all angles in reversed positions of the circle ; the handles that move the slides can be brought down to the eye-end, inside the tube, and consequently made to rotate with it ; and the position circle may be placed at the end of the cradle next the eye- end where it is convenient of access. Struve also points out that by attaching a fine scale to the focussing slide of the eye-piece, and knowing the coefficient of expansion of the brass tube, the means would be provided for determining the absolute change of the focal length of the object-glass at any time by the simple process of focussing on a double star. This, with a knowledge of the tempera ture of the screw or scale and its coefficient of expansion, would enable the change of screw value to be determined at any instant. Or, if we suppose the temperature of the instrument to be the same in all its parts, the changed scale value becomes simply a function of the reading of the focal scale. Bonn It is probable that the Bonn heliometer was in course of con- helio- struction before these suggestions of Struve were published or dis- meter. cussed, since its construction resembles that of the Konigsberg and Pulkowa instruments. Its dimensions are similar to those of the former instrument. Bessel, having been consulted by the cele brated statesman Sir Robert Peel, on behalf of the Radcliffe trus tees, as to what instrument, added to the Radcliffe Observatory, reading the screws or even the heads from the eye-end. Bessel s practice was to unclamp in declination, lower and read off the head, and then restore the tele scope to its former declination reading, the clockwork meanwhile fallowing the stars in right ascension. The setting of both lenses symmetrically would, under such circumstances, be very tedious. 1 Tliis most important improvement would permit any two stars under measure ment caclv to be viewed in tlie optical axis of each segment. The optical centres of the segments would also remain at the same distance from the eye piece at all angles of separation. Thus, in measuring the largest as well as the smallest angles, the images of both stars would be equally symmetrical and equally well in focus. Modern heliometers made with cylindrical slides measure angles over two degrees, the images remaining as sharp and perfect as when the smallest angles are measured. 2 Bessel found, in course of time, that the original corrections for the errors of his screw were no longer applicable. He considered that the changes were due to wear, which would be much lessened if the screws were protected from dust. 3 The tube, being of wood, was probably liable to warp and twist in a very uncertain way. 4 We have been unable to find any published drawing showing how the seg ments are fitted in their cells. 5 We have been unable to ascertain the reasons which led Bessel to choose ivory planes for the end-bearings of his screws. He actually introduced them in the Konigsberg heliometer in 1840, and they were renewed in 1848 and 1850. 6 A screen of wire gauze, placed in front of the segment through which the fainter star is viewed, was employed by Bessel to equalize the brilliancy of the images under observation. An arrangement, afterwards described, lias been fitted in modern heliometers for placing the screen in front of either segment by a handle at the eye-end. 7 This heliometer resembles Bessel s, except that its foot is a solid block of granite instead of the ill-conceived wooden structure that supported his instru ment. The object-glass is of 7 4 inches aperture and 123 inches focus. 8 Description de fObiereatoire central de Pulkowa, p. 208. 9 Steinheil applied such motion to a double-image micrometer made for Strove, This instrument suggested to Struve the above-mentioned idea of employing a similar motion for the heliometer. would probably most promote the advancement of astronomy, strongly advised the selection of a heliometer. The order for the instrument was given to the Repsolds in 1840, but "various circum stances, for which the makers are not responsible, contributed to delay the completion of the instrument, which was not delivered before the winter of 1848. " 10 The building to receive it was com menced in March 1849 and completed in the end of the same year. This splendid instrument has a superb object-glass of 7 inches aperture and 126 inches focal length. The makers availed them selves of Bessel s suggestion to make the segments move in cylin drical slides, and of Struve s to have the head attached to a brass tube ; the eye-piece is set permanently in the axis, and the whole rotates in a cradle attached to the declination axis. They provided a splendid, rigidly mounted, equatorial stand, fitted with every luxury in the way of slow motion, and scales for measuring the displacement of the segments were read by powerful micrometers from the eye-end. 11 It is somewhat curious that, though Struve s second suggestion was adopted, his first was overlooked by the makers. But it is still more curious that it was not afterwards carried out, for the communication of automatic symmetrical motion to both segments only involves a simple alteration previously de scribed. But, as it came from the hands of the makers in 1849, the Oxford heliometer was incomparably the most powerful and perfect instrument in the world for the highest order of micrometric research. It so remained, unrivalled in every respect, till 1873; it remains still, optically, the most powerful heliometer in the world; and, with a few alterations, it might almost rival the most recent instruments in practical convenience and accuracy. These altera tions, all of which could be made without great difficulty, are the following : (a) Beyond the automatic symmetrical motion above-described, the instrument should be fitted with means for adjusting the screens from the eye-end (see footnote 6 in last column). (ft) The arrangement of the scales should be changed. At present both scales are read separately by separate micrometers, each relative to a separate fiducial line. What the observer requires is the difference of the readings of the two scales, and this can obvi ously be most quickly and accurately obtained if the edges of the two scales are brought together, and both are read, relatively to each other, by the same micrometer. (c) The unsatisfactory motion in position angle should be replaced by the action of a pinion (attached to the cradle) in the teeth of a wheel (attached to the tube). 12 (d) The position circle should be read by telescopes or microscopes attached to the cradle, and accessible from the eye-end. (c) It would add greatly to the rapidity of work and the ease of the observer if a small declination circle were attached to the cross- head, capable of being read from the eye-end. As the transit of Venus of 1874 approached, preparations were set on foot by the German Government in good time ; a commission of the most celebrated astronomers was appointed, and it was re solved that the heliometer should be the instrument chiefly relied on. The four long-neglected small heliometers made by Fraunhofer were brought into requisition. Fundamental alterations were made upon them: their wooden tubes were replaced by tubes of metal; means of measuring the focal point were provided ; symmetrical motion was given to the slides ; scales on each slide were provided instead of screws for measuring the separation of the segments, and both scales were read by the same micrometer microscope ; a metallic thermometer was added to determine the temperature of the scales. These small instruments have since done admirable work in the hands of Schur, Hartwig, Kustner, and Elkin. The Russian Government ordered three new heliometers (each of Russian 4 inches aperture and 5 feet focal length) from the Repsolds, and the helio- design for their construction was superintended by Struve, Auwers, meters. Fig. 34. and Winnecke, the last-named making the necessary experiments at Carlsruhe. Fig. 34 represents the type of instrument which re- 10 Manuel Johnson, M.A., Radcliffe observer, Astronomical Observations made at the Radcliffe Observatory, Oxford, in the year 1850, Introduction, p. iii.
n The illumination of these scales is interesting as being the first application of