io8 ELEMENTARY TECHNOLOGY. 



The greatest improvements in modern microscopes relate to 

 the objectives, removing spherical and chromatic aberrations, 

 and collecting the greatest possible amount of light rays from 

 the object observed. The various kinds of objectives are 

 termed air lenses, or dry objectives, water immersion systems, 

 and homogeneous immersion systems, the principles of which 

 have been explained. The line of future improvement will be 

 in the more perfect illumination of the object observed. 



The German opticians usually distinguish their objectives by 

 letters, the French by numbers, and the English and Ameri- 

 can by their equivalent focal lengths, or the focal lengths of 

 lenses to which their amplification corresponds. 



2. Eye pieces. 



The eye piece most generally employed is the Huygenian. 

 It has two plano-convex lenses whose foci areas i to 3, with 

 plane sides next the eye and a diaphragm or stop between 

 them. Another form, called the positive or Ramsdem eye 

 piece, is sometimes used. It is similar to the Huygenian, with 

 the flat sides of the lenses outwards. 



The Orthoscopic eye piece is useful for low powers, since it 

 gives a large field of view. It has a double convex field lens 

 and an achromatic eye lens, but no diaphragm. The Peri- 

 scopic has a triplet eye lens and a double convex field lens. 



A new eye piece, made by Zeiss and by Powell & Lealand 

 is called the Compensating eye piece and used chiefly with 

 the new apochromatic objectives, but serviceable with ordinary 

 achromatics. They have an eye glass of a single plane lens, 

 and a field lens which is an achromatic triplet. As these are 

 numbered according to amplification, it is easy to know what 

 power you are using. Thus a I inch objective=io diameters. 

 With eye piece 12=120 diameters. Or this eye piece with 

 ^ inch objective=48o diameter. These numbers refer to the 

 proper length of tube. Sometimes an amplifier consisting of 

 a concave lens (generally achromatic) is used between the eye 

 piece and the objective and may double the power of the eye 

 piece. 



