GHOSTS AND OCULARS. 51 



the position of the 1st order ghost. The 2n(l order ghost due to 2 

 reflections and one refraction lies at a closely similar 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 with thick lenses the ghosts lie closer in toward the A'ertices, 

 and in achroniats further out. 



3' 



^ 



i F' 



Vl 



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 under scrutiny, and seriously affect the dark adapta- 

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



The ghost focussed 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 

 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 |- / the ghost goes considerably out of 

 focus and appears as a large hazy spot increasing as B moves 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 brilliant. 



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



