On a New Form of Ghost Micrometer. 5 



The attention of one of the authors of this paper (Mr.Grubb) was 

 called to the subject lately, partly by the necessity of devising 

 some special instruments for special work, but more particularly 

 by a letter from Mr. S. W. Burnham, of Chicago, the well-known 

 and successful observer of double stars. At the same time the 

 attention of the other author (Mr. Burton) was called to the same 

 subject by perusal of Prof Kaiser's paper, referred to further on, and 

 on mentioning the special points of the matter to Mr. Grubb, Mr. 

 Grubb was enabled to place in Mr. Burton's hands a model of a 

 micrometer, which he had devised to fulfil some of Mr. Burnham's 

 conditions, made without special reference to the points raised in 

 Prof Kaiser's paper, but which did, nevertheless, meet them. On 

 trial of this model micrometer Mr. Burton found it to work 

 admirably, and since then the two authors, working together, 

 have so far perfected the instrument which you see, bringing it 

 into a convenient and workable form. 



The following short notice, which formed part of a paper read 

 at a recent meeting of the Royal Astronomical Society, will 

 help to convey an idea of the special point of the matter. 



It is well known that the ordinary wire micrometer is unsuit- 

 able for the direct measurement of two classes of objects, namely, 

 planetary discs and double stars near the limit of separability for 

 the object glass employed. 



The difficulties attendant on its use in the cases above mentioned 

 are briefly these — (a) (1.) on bringing a material line in contact 

 with the image of a luminous object of sensible magnitude, 

 diffracted light appears on the side of the wire furthest from the 

 image to be measured ; and (2nd), if a wire be placed on the 

 image of a star, that image becomes elongated in a direction 

 perpendicular to the wire : defect (1) renders it extremely difficult 

 to ascertain when a micrometer wire is accurately in contact 

 with the limb of a planet. — (6) by reason of the interval 

 between components of extremely close doubles becoming filled 

 with light when the wires are placed on the components, it is 

 impossible to obtain direct measures of such objects with the 

 required certainty. 



The difficulties above indicated have been treated very com- 

 pletely by Professor Kaiser, in volume 3 of the Leyden Observations, 

 pp. 104!-5, where he refers with approval to a proposal by Lament 



