6 Scientific Proceedings, Royal Dublin Society. 



(Jahrbuch der K.S. bei Miinchen ; seite 187) to employ ghost lines 

 for such measurements, and by the Rev. W. R. Dawes, in Mem. 

 Royal Astronomical Society, vol. 35, pp. 153 and 161. 



The suggestions of Professor Kaiser seem never to have been 

 carried into effect. 



In the proceedings of the Vienna Academy of Sciences for 1856, 

 vol. XX., page 253, H. Karl v. Littrow describes a mode of forming 

 a "ghost" of a system of lines, applicable and actually applied to a 

 meridional telescope. In this instrument the "ghost" lines are in- 

 terrupted at or near their centres, by the interposition of an opaque 

 bar, and the star is caused to traverse the series of blank spaces 

 thus formed. 



It does not appear, however, that any attempt was made to 

 adapt this to a position micrometer of an equatorial instrument. 



An instrument somewhat resembling that of Dr. Littrov/, and 

 designed for use as a position micrometer was devised by Mr. G. 

 P. Bidder, and is described by him in the Monthly Notices, Roy. 

 Astr. Soc, for June, 1874. In this form of " ghost" micrometer, the 

 image of the lines is formed by a unilateral oblique pencil of rays, 

 and it would, probably, be difficult to avoid loss of definition, due 

 to dispersion by the lenses of the eyepiece, as well as to preserve 

 an invariable scale value, on account of the want of a sufficiently 

 rigid connexion between the several parts of the instrument. Any 

 movement of the second prism of the optical train, as proposed for 

 the purpose of transferring the system of " ghost " lines to any 

 part of the field, would directly tend to alteration of the scale 

 value, and introduce a variable parallax of serious amount. 



The instrument now to be described is one of several forms 

 contrived with the object of avoiding the difficulties encountered 

 by those who have hitherto worked at this subject, and of extend- 

 ing the powers of the wire micrometer into the field occupied 

 by the double image micrometer, by rendering the measurement 

 of planetary discs and close double stars practicable without the 

 production of interference phenomena, or the necessity of 

 halving the light of the image, this last being a great objection to 

 the use of the double image micrometer in the measurement of 

 faint objects. At the same time the connexion between the 

 several parts of the instrument is so rigid and permanent that 

 variation in the scale value will be produced only by tempera- 



