136 ACCESSORY APPARATUS. 



SO as to throw a strong shadow, and either the stage or the illu- 

 minator is made to rotate, so that the light shall fall upon the 

 object successively in every azimuth, information may often be 

 gained respecting the nature of these markings, which can be 

 acquired in no other mode. But the direction given to the rays 

 may be so oblique, that they shall not enter the object-glass at 

 all; in this case, they serve to illuminate the object itself, which 

 shines by the light whose passage it has interrupted ; and as the 

 observer then receives no other light than that which radiates 

 from it, the object (provided it be of a nature to stop enough 

 light) is seen bright upon a dark field. Each of these methods 

 has its advantages for particular classes of objects; and it is ad- 

 visable, in all doubtful cases, to have recourse to every variety of 

 oblique illumination that shall present the object under a diiferent 

 aspect. Almost every Microscopist who has especially devoted 

 his attention to the more difScult lined or dotted objects, has de- 

 vised his own particular arrangement for oblique illumination, 

 and feels confident of its superiority to others. To give a full 

 description of all, would be quite unsuitable to our present ob- 

 ject; those, therefore, will be specially noticed, which have al- 

 ready acquired general approval ; whilst such as have only been 

 recommended by individuals, will be simply referred to. As 

 they have little in common, save their purpose, it seems scarcely 

 possible to classify them according to any other character, than 

 that afforded by the direction which they give to the oblique rays; 

 some of them bi'inging these to bear on the object from one side 

 alone, and others from all sides. 



60. One of the earliest methods devised for obtaining oblique 

 light, was the eccentric prism of M. S"achet ; which, occupying 

 the place of the achromatic condenser, and like it receiving its 

 light from the mirror, has its surfaces so arranged, as to throw a 

 converging pencil of rays on the under side of the object, whose 

 axis is at an angle of about 40° with the axis of the microscope. 

 One great convenience of this instrument lies in the power of 

 giving revolution to the prism, by simply turning it in its socket, 

 so as to direct the oblique rays upon the object from every side 

 successively, without moving the stage. Its principal disadvan- 

 tages consist in the limitation of its aperture (producing a de- 

 ficiency of light), in the want of correction for its chromatic aber- 

 ration, and in the absence of any power of varying the obliquity 

 of the illuminating pencil.' All these disadvantages seem to be 

 remedied by the plan of oblique illumination recently proposed 

 by Mr. Sollitt, of Hull, which consists in the employment of an 

 Achromatic condenser of very long focus and large aperture, 

 mounted in such a manner as to enable its axis to be inclined to 

 that of the microscope through a wide angular range ; a con- 



' A full description of M. Nacliet's prism, and a mathematical investigation of its 

 properties, by iVIr. G. Shadljolt, will be found in the "Transactions of the Microscopical 

 Society" (1st series), vol. iii, p. 74, et scq. 



