8 Scientific Proceedings, Royal Dublim Society. 



are formed by a perforated concave mirror, mounted in a tube 

 capable of being pushed into the draw tube of the telescope. (See 

 Plate III., tig. 6.) The best form of condensing lens L is under con- 

 sideration, and it may possibly be found desirable to superachro- 

 matize it to compensate for the undercorrection of the usual eye- 

 pieces for lateral pencils. 



In two of its simplest forms including that represented in 

 the figure, the ' ghost ' micrometer has been severely tested by 

 one of us (Mr. Burton) as regards the effect of the images of 

 the lines upon the images of various celestial objects formed 

 by an equatorially mounted objective of six inches aperture and 

 six focal length. The objects scrutinised for this purpose were 

 the following : — Lunar details, near the terminator, and fully 

 enlightened ; Jupiter and Saturn, with their satellites ; and 

 numerous stars, both single and double. In all cases the result 

 was the same, not the slightest change of form or false light 

 beyond the limb (of the planets), being visible or even suspected 

 when the objective images were brought in contact with or were 

 occulted (covered) by the ' ghost ' lines, powers up to 400 linear 

 being employed whenever definition was suitable. The illumi- 

 nation of the reticules employed was extremely easy, and readily 

 adjusted to suit the brightness of the object observed. It was 

 possible, with the direct light of an ordinary candle, unas- 

 sisted by any optical arrangement for rendering it parallel or 

 convergent, to increase the brightnes of the lines sufficiently to 

 make them clearly visible against the brightest parts of the moon's 

 image. The reticules employed were systems of lines cut through 

 an opaque film deposited upon glass. 



Suggested forms of " Ghost " micrometers ivith their 

 applications. 



In its most simple form, this micrometer will do duty as a 

 transit eyepiece displaying dark lines on a bright field, or r. r., 

 and can be fitted with reticules of any kind for mapping 

 groups of stars, or details of the moon or planets, including of 

 course circles for ring micrometer work. 



(]) By substituting for the reticule a wire micrometer with 

 one or two screws, the instrument can be used (a) instead 

 of an ordinary wire — or bifilar micrometer with bright field, 



