On a New Form of Ghost Micrometer. 7 



ture, and the corrections thereby necessitated will be of the 

 same order with those already familiar to observers. 



Plate III., figure 5, represents the most simple form of the in- 

 strument hitherto devised by us, one-third of its actual size. Ex- 

 ternally it consists of a pair of tubes, one in the axis of the 

 telescope, and the other at right angles thereto. The first of 

 these tubes is attachable to the draw tube by an ordinary 

 ' Tulley ' screw at S, and carries at its lower end the eyepiece E. 

 The cross tube carries at one end a ' Reticule ' or wire frame F, 

 movable or stationary, according to requirements, and at the 

 other a cell containing a concave silvered mirror M, the radius 

 of curvature of which is slightly greater than the distance 

 between F and M. At the junction of the tubes is mounted (at 

 45° to F. M) a plane silvered mirror PP, perforated by an ellip- 

 tical hole slightly larger than is necessary to allow the pencil of 

 rays from the object glass to pass. Immediately below PP is in- 

 serted a perforated achromatic lens LL to increase the conver- 

 gence of the rays from M, thus diminishing the length of the eye 

 tubcj while forming an image of F at the focus of the object 

 glass. F being illuminated by a faint external light the rays 

 from it pass to M through the hole in PP, and being reflected by M 

 in a cone of smaller angles are received on the inner edge of PP, 

 and are thereby diverted into the eye-tube where they are still 

 further condensed by LL and brought to a focus at F — the focus 

 of the object glass. 



In Plate IV., fig. 7, is shown the original form which Mr. 

 Grubb placed for trial in Mr. Burton's hands. The principle is 

 precisely the same, but greater compactness is obtained by bending 

 back the rays as in Plate III., figure 5. 



In some cases the lens L (Plate III., fig. 5) may be unnecessary, 

 in others it may be found better to use a plane mirror at M, and 

 throw the whole work of forming the ' ghost ■" images on the lens 

 L. It is, of course, not necessary that the cross tube should be at 

 right angles to the eye tube ; and, in some casts, it may be ad- 

 visable to use different arrangement of the parts. If it should 

 prove to be diflicult to adapt the instrument to any existing 

 telescope on account of its length (4 inches), a Barlow lens can 

 be inserted at S ; but some forms of the apparatus have been de- 

 vised, in which there is no cross tube, and the ' ghost ' images 



