On a New Form of Ghost Micrometer. 3 



constantly and carefully, the lamps, not being vertical, allow the 

 oil to escape over the micrometer and person of the observer in a 

 highly objectionable manner. So great is this nuisance that 

 almost invariably observers try and work with one lamp ; but this 

 is essentially bad, giving a one-sided illumination to the wires, 

 and thereby causing serious errors of estimation. 



The next plan we would notice is that of surrounding the 

 system of wires by a thin, translucent substance such as a very 

 thin tube of ivory, and rendering this luminous by light thrown in 

 any direction. This plan answers tolerably well for a faint illu- 

 mination; but it is difficult to obtain sufficient brilliancy for any 

 save faint objects, and if any considerable brilliancy is obtained it 

 is almost impossible to prevent the field lens of the eye-piece 

 becoming luminous and destroying the blackness of the field. As 

 before stated, however, it can be made to answer for faint objects, 

 and this plan has occasionally been used by one of the authors of 

 this paper (Mr. Grubb), when other plans were not conveniently 

 adaptable, as in the case of the Dunsink Refractor. 



A third plan is that for which arrangements have been made in 

 the Vienna Telescope, but which, so far as it goes, has frequently 

 been used before (see Plate 1, fig. 1). Four prisms or reflectors 

 are placed in the telescope tube opposite the Dec. axis, which 

 prisms direct the light down through four tubes placed in the 

 interior of the main telescope tube. In the diagram (Plate 1, 

 fig. ] ), are two of these four prisms (the other two could not 

 be shown with convenience). These four beams of light are 

 then received on four small reflectors, two of which are shown 

 in diagram, (r. r.) These reflectors surround the wire plate, and 

 the light reflected from these mirrors passes across the wires at 

 such a considerable angle as prevents any danger of the fleld lens 

 being illuminated or the blackness of the field injured. 



A fourth plan is shown in Plate 1, figure 2, and is that adopted 

 generally by one of the authors (Mr. Grubb) for his smaller in- 

 struments. 



A prism, P, on outside of telescope tube receives light from a 

 swinging lamp and reflects it down through a small tube attached 

 to outside of main telescope tube, where it is thrown on a reflector, 

 R. This reflector, R, is carried on the end of a T tube which is 

 capable of revolution round the tube which carries the micro- 



SciEN. Proc, R.D.S. Vol, hi., Ft. i. B 2 



