ILLUMINATION.] M I C E O M E T E 11 249 its light into the tube, D a disk to regulate the quantity of light, B a disk with glasses to regulate the colour of the light, S a spring to clamp the disks, C the counterpoise of the lamp, G a poise to preserve the horizontality of the axis CL. But astronomers owe to the genius of Grubb the introduction of a more efficient and convenient system, viz., the performance of all necessary illu mination of an astronomical telescope by a single lamp, and the perfect control of the illumination of the field or webs, and the regulation of these as to intensity or colour by simple motions from the eye-end. It is impossible to speak too highly of Grubb s efforts in this direction ; he has broken the ground in this department of astronomical engineering, and rendered the working of so huge an instrument as the Vienna telescope of 27 inches aperture not only convenient, but easier for a single observer than that of a very small telescope of the older constructions. But in the illumination of the field wires and scales of a micro meter Grubb s original method has recently been surpassed by one which is due to the Repsolds. We shall therefore describe the latter. Fig. 20 represents the eye-end of a telescope. The reader will recognize the micrometer (figs. 16 and 17) previously described. L is a paraffin lamp fitting by a bayonet joint into a copper cover c. This effectually defends its glass chimney against accident, and protects the lamp from wind. The simple means by which this lump is made to preserve its ver- ticality in all positions of the telescope is evident from the figure. By this lamp alone the bright wire or bright field illumi nation is given at pleasure, and with any desired intensity, simply by movement of the small pin p. The position circle and the head of the micrometer are also illu minated, as well as the declina tion circle, by the same lamp. AB is a cylindrical box, ending in a truncated cone towards A. It is shown, mid-section, in a plane passing through the telescope axis, in fig. 21, where all details un necessary to the explanation of the illumination are omitted, and pro portion of parts is sacrificed to clearness. P is a prism (fig. 21) that rotates with the lamp and reflects its light into AB. The flame of the lamp is in the focus of the lens II, so that the rays become parallel after passing through it. There is a sliding motion to perfect this adjustment. annular reflector of speculum metal rr (fig. 21), which reflects light upon the double mirror M (fig. 20), whence it is diverted to the two opposite points on the declination circle that are read by micrometer microscopes from the eye-end (the latter are omitted for sake of clear ness). The little handle at p and the dotted lines p z represent an iris-diaphragm, very ingeniously constructed, mount ed on a plate of transparent glass. There is a flat ring of brass, carrying four pins, which is turned by the handle p , in a plane at right angles to Pra. These pins work in spiral slots cut in four slides. Thus Fig. 20. There is a well-polished flat 21 - rotation of the ring causes the four slides to approach or recede from a centre. When the handle p is in the middle of its range, the slides together form a disk as large as the hole in the diaphragm dd, and thus prevent all light from entering the telescope tube. When P is pushed to one side of its range the slides move outwards leaving a square opening in the centre so that the light falls on the prism n, whence it is diverted to a silvered reflector cemented on the middle of the inner surface of the object-glass, and is then reflected back along the axis of the telescope to illuminate the field at u. When p is pushed to the other side of its range the slides approach and overlap at the centre, excluding light from n and allowing it to fall upon the reflector 5 instead. From 5 the light is thrown upon the webs to, u by reflexion from a white papier mache surface laid on the inside of a thin hollow brass truncated cone xx. The edge of this cone forms the circle seen within TT in fig. 17. All stray light is prevented by the light-guard tube mm, which is attached to and moves with the rotating part of the micrometer. The result is to produce a symmetrical illumination of the whole system of webs in a perfectly dark field. It is also obvious that by placing p at an intermediate position between the centre and the extremes of its range any desired modification of bright wire or bright field illu mination can be obtained at pleasure. The light falling on the papier mache hollow cone is intercepted at three points by prisms, one of which ^ is shown in section. These prisms are inserted in the cylinder which carries the foundation plate of the micrometer box and rotate with it. Two of them divert light upon the reflectors (seen from different points of view in figs. 16, 17, 20). The third prism after two reflexions (figs. 16, 20) illuminates the micrometer head. The whole arrangement is in the highest degree elegant, and we have found it most simple and convenient in practice. The screen C (tigs. 20 and 21) made of thin copper and attached to AB effectu ally protects the observer s eye from stray light from the lamp. It has been found essential, in bright field illumination, when the highest accuracy is desired, to have the illuminating rays parallel with the telescope axis. In the best telescopes of the future some plan like that of Rep- sold s, above described, will doubtless be adopted. It is probable also that with the introduction of condensers, in conjunction with the incandescent carbon light in vacuum, electricity will ultimately supersede the oil or paraffin lamp in illuminating astronomical instruments. A small " Swan lamp " can be placed anywhere, is unaffected by wind, and gives off comparatively little heat. These are most valuable qualities for the purpose in question. The astronomer-royal (Mr Christie) has recently used luminous paint to render the measuring pointer of the Greenwich spectroscope visible at night. This paint, after exposure during the day to sun light, shines at night with a dull phosphorescence sufficient to make the micrometer pointer, to which it is applied, faintly visible, and, it is stated, with very satisfactory results. On the use of the filar micrometer consult Stnivc, Menswx Micrometricx, St Petersburg. 1837; Brunnow, Practical and Spherical Astronomy; Chauvene), Practical and Spherical Astronomy; Bninnow, Astronomical Observations and Researches made at Ounsink, Dublin, 1870, 1873, 1879; Ball, ibid.; Kaiser, Leiden Observations; und the papers of Dembowski in the Astronomische Nachrichten. Double-Image Micrometers. The discovery of the method of making measures by double images is stated to have been first suggested byRoemer about 1678. Roemer. But no such suggestion occurs in the Basis Astronomise of Horre- bow (Copenhagen, 1735), which contains the only works of Roemer that remain to us. It would appear that to Savary is due the first invention of a micrometer for measurement by double image. His heliometer (describ ed in a paper com municated to the Royal Society in 1743, and printed, along with a letter from Short, in Phil, Trans., 1753, p. 156) was constructed by cutting from a com plete lens abed the equal portions aghc and acfe (fig. 22). The segments gbh and efd so formed were then attached to the end of a tube having an internal diameter represented by the dotted circle (fig. 23). The width of each of the portions aghc and acfe cut away from the lens was made slightly greater than the focal length of lens x tangent of sun s greatest dia meter. Thus at the focus two images of the sun were formed nearly in contact as in fig. 24. The small interval between the adjacent limbs was then measured with a wire micrometer. Savary also describes another form of heliometer, on the same Savary. principle, in which the segments aghc and acfe are utilized by cementing their edges gh and tf together (fig. 25), and covering all except the portion indicated by the unshaded circle. Savary ex presses preference for this second plan, and makes the pertinent remark that in both these models "the rays of red light in the two solar images will be next to each other, which will render the sun s disk more easy to be observed than the violet ones. " This XVI. v Fj 29
Fi" 24.