117 



There are a large number of other eyepieces 

 called by a variety of names which can be found 

 in catalogues, but for which there is very limited 

 use. They, therefore, require no special mention 

 in this connection. 



Compensating Eyepiece. In this place it is 

 .also proper to speak of the new compensating eye- 

 piece as made and suggested by Prof. Abbe. 

 These eyepieces compensate the residual errors in 

 the apochromatic objectives and are of no value 

 except when used with them and for projection, as 

 they are highly over-corrected, as can be seen in 

 the edge of the field, which has a strong orange 

 color, whereas the Huyghenian has a blue color. 



In this list of eyepieces there is a searcher which 

 is of low power and intended to find objects. The 

 higher powers are for general work, some of them 

 being negative others positive. One more kind is 

 the projection eyepiece which is intended for pro- 

 jecting an image on a photographic plate, or on a 

 screen or wall. 



Rating or Designation. Until within recent 

 years the rating of eyepieces was arbitrary, each 

 maker following a system of his own and while 

 this is still done, the general method followed is to 

 name them as in objectives, according to a single 

 lens, which the equivalent focus of the eyepiece 

 lenses will equal and do this in inches or milli- 

 meters. This conveys an idea of the extent to 



