THE MICROSCOPE. 87 



may be united into one, forming the solid eye-piece. The 

 latter is the better and is at the same time more expensive 

 form. It is best to have two eye-pieces (those lettered A and 

 C by most American manufacturers being the most useful). 

 The greatest attention should be paid to the selection of the 

 objectives and, unless the purchaser be an experienced per- 

 son, some expert friend should, be called in to pass judgment 

 upon the lenses submitted. The great points to be secured 

 are absence of color, flatness of field, and distinctness of 

 image. All objectives above a one-fourth should be provided 

 with an adjustment for cover glass. The " angle of aperture " 

 should be reasonably large, but "high angled" lenses possess 

 no value corresponding to their high price except to "Diato- 

 maniacs." 



Objectives are rated by English and American manufacturers 

 according to their focal length, this being the distance be- 

 tween the object and the "optical centre" of the lens when 

 an image is formed. Thus a inch does not have a quarter 

 of an inch between the objective and the object, but between 

 the optical centre (which is nearer the distal portion of the 

 lens than is the mathematical centre) and the object. 



For the beginner, the most useful objectives will be an 

 inch and a quarter or fifth. As he proceeds in his studies and 

 his familiarity with the instrument increases, higher, lower, 

 and intermediate powers will be necessary. 



Among the higher powers ( inch and upward) it is cus- 

 tomary to have the lenses of the kind known as "immersion." 

 In these the end of the objective is wetted with a drop of 

 water which forms a thin film between the cover of the slide 



