GHOSTS AND OCULARS. 51 



the position of the 1st order ghost. The 2nd order ghost (hie to 2 

 reflections and one refraction lies at a closely siniihir distance, /' to 

 the rear. The loci can be readily found by holding a large lens at a 

 fair distance from a lamp and looking for the ghosts with a slip of 

 ground glass. The position of these 2nd order ghosts is not greatly 

 changed by distributing the total curvature o^■er both sides of the 

 lens, while Avith thick lenses the ghosts lie closer in toward the vertices, 

 and in achromats further out. 



3 



', I 



f F' \ jC' 



\ I 



Fig. 2. 



It is this 2nd order field-lens ghost that is liable to cause the most 

 trouble. In a 2-lens ocular some 10% of the whole incident light is 

 scattered behind the stop, which may be a large quantity compared 

 with the object inider scrutiny, and seriously affect the dark adapta- 

 tion of the eye. A variable stop, could often be used to advantage. 



The ghost f ocussed at /' lies inside the focus F' of the ocular and is 

 visible as a somewhat blurred spot the more conspicuous the nearer 

 to focus. In a Ramsden ocular of the theoretical form / = f = F 

 the ghost is very obtrusive. 



For example, in the rather thick field lens of a big positive ocular I 

 found the rear ghost at less than yV ^^^^ focal length back of the 

 rear vertex. And a very little experimenting with a pocket lens will 

 enable one to realize that such a ghost is not out of focus enough to 

 count. As the ocular is shortened by moving the eye lens B to the 

 conventional separation of f / the ghost goes considerably out of 

 focus and appears as a large hazy spot increasing as B moA'es toward 

 B'. An ocular with sliding fit between field and eye lens gives a very 

 instructive view of the way the ghost comes to approximate focus as 

 the lenses are separated, especially if one half-silvers a surface of the 

 field lens, when the effects are very Ijrilliant. 



The best place to study ghosts is in an ordinary prism glass where a 



