22 PRACTICAL HISTOLOGY. 



is greater than the outward deflection ; and' so, by a careful 

 adjustment of the angles of the prisms, the dispersive 

 power of/ can exactly reverse that of c, while the deflec- 

 tion is not overcome, hence the ray r emerges from / 

 deflected but not dispersed. The system is achro??iatic. An 

 achromatic object-glass may consist (Fig. 25) of a bi-convex 

 lens of crown, and a plano-concave lens of flint glass 

 cemented together with Canada balsam. The effect is the 

 same as that of the prisms in Fig. 24. Luminous rays, 

 whether travelling to or from the point F, leave the lens 

 achromatic. 



THE OPTICAL PARTS OF THE COMPOUND 

 MICROSCOPE. 



35. The Objective. The value of a compound 

 microscope depending principally on the quality of the 

 objective, it is usually termed the lens. It consists of one 

 or more compound lenses (O, Fig. 21). In objectives of 

 medium power, e.g. -inch focus, it has been generally the 

 fashion to construct them of three compound plano-con- 

 vex achromatic glasses. Recently, however, Mr. Wenham 

 (Proc. Roy. Soc.vol. xxi. p. in) has devised an objective, 

 in which only one concave lens of dense flint glass is used 

 to correct four convex lenses of crown glass. By this 

 means the cost of the production of lenses of high quality 

 has been much lessened. 



36. The powers of continental lenses are usually dis- 

 tinguished by the numbers i, 2, 3, and so on; in this 

 country, their powers are indicated by their focal length. 

 The lenses most useful for the beginner are the i-inch and 

 ^--inch ;* giving, with a suitable ocular, a magnifying power 

 of about 50 and 300 diameters. Much higher powers in 

 addition to these are required by the advanced student, 

 such as T V, tV an d ITS or nV A sir nas been constructed, 

 but for ordinary purposes, the -^V is as high a power as need 

 be employed. The lenses made by Hartnack and by 

 Zeiss, on the Continent, are commendable on account of 



* These have a power similar to objectives 3 and 7 of Hartnack's 

 microscope. 



