8 MICROSCOPICAL DIAGNOSIS. 



It is not made of a single glass because the powers of refraction 

 and dispersion are not equally united in any single refracting 

 medium. Now crown and flint glass act with regard to each other 

 in such a manner that if a crown glass lens be united with a flint glass 

 lens, the refraction of the former will be lessened by the dispersive 

 action of the latter, while the color dispersion of the former is neutral- 

 ized by the opposite action of the latter. Thus by combining these 

 glasses the aberrations are remedied to a large extent. 



In the case of our figure the lower plano-concave lens is made 

 of flint glass, and the upper bi-convex lens of crown glass. These 

 two glasses constitute, in this case, a system, and in fig. 2, three of 

 these systems complete the objective. These systems are mounted 

 in either brass or hard rubber, which, at its upper extremity, is pro- 

 vided with a screw of standard size. Such a sized screw is termed 

 a "society screw." All the first-class stands are so made that the 

 body will receive any objective provided with the standard screw. 





2. 



A, Achromatic objective of three systems ; B, objective with high angle ol aperture ; C, ob- 

 jective with low angle of aperture. The angle of aperture is the angle c a c. 



When estimating the comparative value of different objectives 

 the following "good qualities" should be considered: 



1. Defining power. 



2. Resolving power. 



3. Working distance. 



4. Flatness of field. 



Defining power is, without question, the most important quality 

 to be sought in a lens. Its presence makes the objective of great 

 value and its absence renders it simply worthless. It depends upon 

 the completeness of the corrections for spherical and chromatic 

 aberrations, especially the former. Defining power gives a clear, 

 distinct and sharp outline to the image. Its absence is denoted by 

 haziness, indistinctness and a general want of clearness. As 



