Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/270

Rh 254 MICROMETER [HELIOMETEK. suited from their labours. The brass tube, strengthened at the bearing points by strong truly-turned collars, rotates in the cast- iron cradle q attached to the declination axis, a is the eye-piece fixed in that axis, b the micrometer for reading both scales. Fig. 35. c, d are telescopes for reading the position circle p, e the handle for quick motion in position angle, /the slow motion in position angle, g the handle for changing the separation of the segments by acting on the bevel-wheel g&quot; (fig. 35). h is a milled head con nected by a rod with h (fig. 35), for the purpose of interposing at pleasure the prism IT in the axis of the reading micro meter ; this enables the observer to view the graduations on the face of the metallic thermometer TT (composed of a rod of brass and a rod of zinc), i is a milled head connected with the wheel i i (fig. 35), and affords the means of placing the screen s (fig. 34), counterpoised by w over either half of the object-glass. k clamps the telescope in declination, n clamps it in right ascension, and the handles in and I provide slow motion in declination and right ascension respectively. The details of the interior mechanism of the &quot;head&quot; will be almost evident from fig. 35 without description. The screw, turned by the wheels at g, acts in a toothed arc, whence, as shown in the figure, equal and opposite motion is com municated to the slides by the jointed rods v, v. The slides are kept firmly down to their bearings by the rollers r, r, r, r, attached to axes which are, in the middle, very strong springs. Side- shake is prevented by the screws and pieces k, k, k, k. The scales are at n, n ; they are fastened only at the middle, and are kept down by the brass pieces t, t. A similar heliometer was made by the Repsolds to the order of Lord Lindsay c for his Mauritius expedition in 1874. It dilfered only from the three Russian in struments in having a mounting by the e Cookes in which the declination circle reads from the eye-end. 1 This instru ment was afterwards most generously lent by Lord Lindsay to Gill for his ex pedition to Ascension in 1877. 2 These four Repsold heliometers proved to be excellent instruments, easy and convenient in use, and yielding results of very high accuracy in measuring dis tances. Their slow motion in position angle, however, was not all that could be desired. When small movements were communi cated to the handle e (fig. 34) by the tangent screw/, acting on a small toothed wheel clamped to the rod connected with the driv ing pinion, there was apt to be a torsion of the rod rather than an immediate action. Thus the slow motion would take place by jerks instead of with the necessary smoothness and certainty. When the heliometer part of Lord Lindsay s heliometer was ac- 1 For a detailed description of this instrument see Dunecht Publication!:, vol. ii. 2 Mem. Royal Astronomical Society, vol. xlvi. pp. 1-172. quired by Gill in 1879, he changed the manner of imparting the motion in question. A square toothed racked wheel was applied to the tube at r (fig. 34). This wheel is acted on by a tangent screw whose bearings are attached to the cradle ; the screw is turned by means of a handle supported by bearings attached to the cradle, and coming within convenient reach of the observer s hand. The tube turns smoothly in the racked wheel, or can be clamped to it at the will of the observer. This alteration and the new equa torial mounting have been admirably made by Grubb ; the result is completely success ful. The instrument so altered has been in constant use at the Cape Observatory since March 1881 in determining the paral lax of the more interesting southern stars. Still more recently the Repsolds have com- Yale pleted a new heliometer for Yale College, College New Haven, United States. The object-glass helio- is of 6 inches aperture and 98 inches focal meter, length. The mounting, the tube, objective- cell, slides, &c. , are all of steel. 3 The in strument is shown in fig. 36. The circles for position angle and declination are read by micrometer microscopes illuminated by the lamp L; the scales are illuminated by the lamp L T is part of the tube proper, and turns with the head. The tube Y, on the contrary, is attached to the cradle, and merely forms a support for the finder Q, the handles at/ and p, and the moving ring P. The latter gives quick motion in position angle ; the handles at p clamp and give slow motion in position angle, those at / clamp Fig. 36. and give slow motion in right ascension and declination. a is the eye-piece, b the handle for moving the segments, c the micro meter microscope for reading the scales and scale micrometer, d the micrometer readers of the position and declination circles, e the handle for rotating the large wheel E which carries the screens. The hour circle is also read by microscopes, and the instrument can be used in both positions (tube preceding and 3 The primary object was to have the object-glass mounted in steel cells, which more nearly correspond in expansion with glass. It became then desirable to make the head of steel for sake of uniformity of material, and the advantages of uttel in lightness and rigidity for the tube then became evident.