CHAPTER III. 



EYE-PIECES AND OBJECTIVES. 



Eye-pieces or Oculars. When the student purchases a 

 microscope stand, he will generally find it supplied with 

 the lowest power Huyghenian or negative eye-piece, usually 

 designated by the letter A. At the same time, it may be 

 stated that others, possessing greater degrees of amplifica- 

 tion are often substituted or added at the wish of the 

 purchaser ; and it should be remembered, in the selection 

 of a microscope stand, that the eye-pieces of one maker 

 will not always fit the tubes made by another. It is a 

 thousand pities opticians have not yet learned that their 

 time may be more profitably occupied than by making 

 adapters for each other's instruments. 



The Huyghenian eye-pieces or " oculars " of low power 

 are generally styled "shallow," to distinguish them from 

 those which give greater amplification, which are called 

 " deep " the terms deep and shallow being applied to the 

 degree of curvature possessed by the lenses employed in 

 their construction, and not to the distance between them, 

 as some writers have stated. 



A full-size section of the Huyghenian A eye-piece is 

 shown in Fig. 25, so that the student may understand the 

 details of its construction. 



It consists of two plano-convex lenses, placed at a dis- 

 tance from each other equal to half the sum of their focal 

 lengths, the best proportion of relative radii being i to 3. 



