10 PRACTICAL HISTOLOGY. 



The following patterns of microscopes are to be commended : 



Leitz, Stands J^"os. V., III. 17, IV. 19, V. 23, with objectives 



3 and 7. 

 Zeiss, Stands Nos. VI. and VII., with objectives C and E. 



9. Eye-Pieces or Oculars. Two eye-pieces are required, one 

 medium length and the other shorter, the latter being the more 

 powerful. The two most useful eye-pieces are Nos II. and IV. of 

 foreign makers (of Leitz, however, I. and III.), or A and C of 

 English makers. English makers speak of a deep and a shallow eye- 

 piece ; the former (IV. or C) is shorter, and is the more powerful ; 

 the latter (II. or A) is longer, and is a weaker eye-piece. 



It is to be remembered that the eye-piece only magnifies the 

 image of the preparation formed by the objective, and does not 

 magnify the preparation itself. Hence any fault in the lens, and 

 consequently in the image in the tube, is magnified by the eye- 

 piece. Moreover, the field is not so bright as with a weaker eye- 

 piece. Hence it is expedient to use rather a weak eye-piece. There 

 is an exception to this rule in the case of apochromatic lenses (p. 13), 

 which yield a magnification of the object by means of special eye- 

 pieces, without any loss in the brightness or sharpness of the image. 



These comprise the essential parts of the microscope, but if 

 expense be no objection, the stand may be provided with a hinge- 

 joint, which enables the microscope to be inclined at any angle (figs. 

 10, 14). In some microscopes the tube can be elongated by a draw- 

 tube (fig. 14), which, when it is elongated, increases the magnifying 

 power of the instrument. It is well to have the draw-tube with a 

 scale engraved on it, as is done in the more expensive instruments. 



10. Sub-Stage Condenser. When working with high powers, this 

 is essential, more especially in connection with bacteriological work. 

 Abbe's condenser is by far the most convenient form. Fig. 14 shows 

 one of the more expensive stands fitted with a sub-stage condenser. 



The essential feature of Abbe's illuminating apparatus is a con- 

 denser system of very short focus, which collects the light reflected 

 by the mirror into a cone of rays of very large aperture and projects 

 it on the object (fig. 1 3). 



The cone of light is usually reduced by diaphragms of suitable 

 size, or by means of an iris diaphragm. By means of the rack-work 

 the diaphragm can be placed excentrically and oblique illumination 

 obtained. This apparatus can be used for ordinary work, but it is 

 specially useful for the investigation of bacteria. The rays of light 

 from the condenser are brought to a focus in the object, so that an 

 enormous amount of light is concentrated on the object. The angle 

 of aperture of Abbe's condenser is 120. The full aperture of the 

 illuminating cone of rays is only used when observing deeply-stained 



