112 APPENDAGES TO THE MICROSCOPE. 



81. Oblique Illuminators. — It is frequently desirable to obtain 

 a means of illuminating Transparent objects witb rays of more 

 obliquity than can be reflected to them from the Mirror, even 

 ■when this is thrown as much as its mounting will permit out of 

 the axis of the Microscope ; or than can be transmitted by the 

 ordinary Achromatic Condenser, even when all but its marginal 

 aperture is stopped- out. Such oblique light may be used in two 

 entirely different modes. — The rays, although very far out of the 

 axis of the Microscope, may still not make too great an angle with 

 it to fall beyond the aperture of the Objective ; and thus, entering 

 its peripheral portion after their passage through the object, they 

 will form the image in the ordinary way. The advantage of such 

 oblique illumination arises from its power of bringing-out markings 

 which cannot be seen when only direct rays are employed ; and 

 when the rays come only from one side, so as to throw a strong 

 shadow, and either the Stage or the Illuminator 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, such as can be acquired in no other mode (§ 119). 

 — 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 advisable, in all doubtful cases, to have recourse 

 to every variety of oblique illumination that shall present the 

 object under a different aspect. Almost every Microscopist who 

 has especially devoted his attention to the more difficult lined or 

 dotted objects, has devised his own particular arrangement for 

 Oblique Illumination ; but those methods only can here be noticed 

 which have acquired general approval.* 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 bringing 

 these to bear on the object from one side alone, and others from 

 all sides. 



82. The Amid Prism, which causes the rays to be at once 

 reflected by a plane surface and concentrated by lenticular sur- 

 faces, so as to answer the purpose of Mirror and Condenser at the 

 same time, is much approved by many who have used it. Such a 

 Prism may be either mounted on a separate base, or attached to 

 some part of the Microscope -stand. The mounting adopted by 

 Messrs. Smith and Beck, and shown in Fig. 62, is a very simple 



* Various other methods will be found described in the successive 

 volumes of the "Transactions of the Microscopical Society" and of the 

 " Quarterly Journal of Microscopical Science." 



