36 , PRACTICAL HISTOLOGY. 



FIG. 33. Course of the rays through Hartnack's 

 binocular eye-piece. (Ranvier.) 



50. Dissecting Microscopes. The simple dissecting micro- 

 scope devised by Darwin (Fig. 34) answers very well for ordinary pur- 

 poses. Many, however, will prefer the binocular dissecting microscope 

 made by R. and J. Beck, or Lawson's binocular dissecting microscope 

 made by Collins. 



A Briicke's lens (Fig. 35) is a very convenient form of simple 

 microscope for assisting in dissecting and preparing microscopical 

 specimens, and in making fine drawings. It consists of two plano- 

 convex lenses, with their convex surfaces facing each other, and a con- 

 cave lens above all. This arrangement increases the working distance 

 between the microscope and the object. It has been adopted by R. 

 and J. Beck in their binocular dissecting microscope. 



ACCESSARY APPARATUS FOR ILLUMINATION. 



51. Lamp. If good daylight cannot be obtained, a 

 lamp is necessary. Artificial light is far more fatiguing to 

 the eyes than daylight, on account of the predominance of 

 yellow rays. These may, however, be removed by trans- 



