On a New Form of Ghost Micrometer. 9 



or (b) as a nnifilar micrometer with bright lines cut through 

 an opaque film ; or (c) as a bifilar micrometer with bright lines, 

 the object lines (at F), being in this case spider webs, illumi- 

 nated by a Wenham paraboloid or other similar contrivance. 



(2) By splitting the plane mirror P centrally in the plane M.P.E., 

 and mounting one-half on an axis perpendicular to that 

 plane, attached at its upper end to a lever arm terminating 

 in a sector actuated by an endless screw with divided head, 

 a double set of " Ghost " lines might be formed by inclining the 

 two parts of the plane P to another by a movement of the 

 arm screw. One of these sets of lines would be stationary, 

 while the other set would have its movement registered 

 by the turns and parts of a turn of the endless screw. 

 Of course the lines might be either dark on a bright field 

 or bright on a dark field, the latter being best ; for in 

 the former case, the lines would not be black, but would 

 be of half the brightness of the field. We have experi- 

 mentally proved that the definition of the lines is practically 

 unafiected by stopping ofi" half the aperture of M or P, in a 

 direction perpendicular to them, and that their brightness does 

 not sensibly diminish for a considerable distance past the 

 centre of the field. 



(3) By substituting diaphragms pierced with small circular 

 apertures, as in the instrument of Professor Stampfer (vide Note 

 at end), illuminated by polarized light variable in intensity and 

 colour, this micrometer can be made available for photometric 

 and colour comparisons of stars after the method of ZoUner, and 

 not only can the field, in the case of bright field illumination, 

 be illuminated with light of any desired tint, but, if bright 

 lines are employed, they can be coloured as the observer may 

 prefer, for there is always a superabundance of light, which is 

 yet completely under control. The lines can be shown broken 

 or continuous as may be desired. The first mentioned arrange- 

 ment may be advantageous under some circumstances. 



A useful modification of the reticule mentioned above, would 

 be a bright ring (photographed), divided into numbered degrees, 

 and nearly of the same diameter as the field, which would assist 

 materially in estimation of position angles when the faintness 

 of the object viewed rendered measurements impossible. 



