44 



the two lenses (at the diaphragm) in contradistinction 

 to a positive, in which the focal point is outside of 

 and below the field-lens. 



The Continental eye-piece is also a Huyghenian, 

 although it is mounted in a straight tube, in place of 

 the mounting, with the neck as shown in cut. The 

 American Society of Microscopists has recently 

 recommended that eye-pieces be made par-focal, that 

 is, that the equivalent foci coincide and that the par- 

 focal plane correspond with the upper end of the 

 tube. This is an excellent plan, as the focus on the 

 object is maintained whatever may be the change in 

 eye-pieces. 



Solid Eye-Piece. This was the invention of the 

 late R. B. Tolles, and also belongs to the class of 

 negative eye-pieces. It is called solid 

 from the fact that instead of being 

 composed of two lenses, it consists 

 of one piece of glass, which is cut to 

 a cylindrical form, and on the ends 

 of which the proper curvatures are 

 ground ; the diaphragm is made by Fig. 13. 



cutting a circular groove into the glass at the proper 

 distance between the two surfaces, which is then filled 

 up with an opaque pigment. 



These eye-pieces are only made in high powers, as 

 optical glass is usually not of sufficient homogenuity 

 to make low powers, and their cost would be too con- 

 siderable, without a corresponding advantage. For 

 high powers they are superior to the Huyghenian, in 



