OBLIQUE ILLUMINATION. 



159 



Fig. 



have recourse to the Accessories which are specially provided to 

 furnish oblique illumination in the most effectual manner. Thus 

 by using the Webster Condenser (§ 80) or an Achromatic Condenser 

 of large angular aperture (§79) with a central stop, rays of great 

 obliquity are admitted from every azimuth at once ; and there are 

 some objects which are 

 best seen in this manner. 

 Either of these condensers, 

 again, may be used, like 

 Mr. Reade's Hemispherical 

 Condenser (§ 83), with 

 diaphragms that allow light 

 to pass only from some 

 particular portion or por- 

 tions of their periphery; 

 thus illuminating the ob- 

 ject from- the exact direc- 

 tion or directions best 

 adapted to develop its 

 markings. And if the 

 Stage of the Microscope 

 should not be capable of 

 rotation in the optic axis 

 of the instrument, the 

 required variety of direc- 

 tion may be given by 

 rotating the eccentric 

 Diaphragm. In most first- 

 class Microscopes this rota- 

 tion can be given to the 

 Illuminating apparatus by 

 a rack-and-pinion move- 

 ment. Very oblique illu- 

 mination in one direction 

 only may also be conve- 

 niently obtained by the use 

 of the Amici Prism (§ 82), 

 which combines the action 

 of Mirror and Condenser, 

 and which maybe rendered 

 still more effective by being 

 made achromatic ; and 

 where it is desired to 

 bring out simultaneously 

 two sets of lines crossing 

 each other transversely or 

 obliquely, two such prisms may be employed at once, so fixed as to 

 throw the light of two separate lamps in the most advantageous 



Valve of Pleurosigma Formosum,~with 

 portions a, b, c, d, showing diverse effects 

 of Illumination. 



