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XVI. — On the Illumination of Transparent Microscopic Objects on 

 a New Principle. By F. H. Wenham, Esq. 



(Read April 17, 1850). 



For the purpose of obtaining the perfect definition of microscopic 

 objects under high powers, it is important that the light should be 

 achromatic, and that the illuminator or condenser should transmit a 

 large angular pencil of rays; also the markings on test-objects are 

 in most instances more readily discovered by throwing the light on 

 them obliquely. 



In the prism of Nachet, which is especially intended for oblique 

 illumination, the attempts made to obtain a large aperture, and to 

 correct the chromatic aberration, have not been successful; conse- 

 quently there is a want of light, and objects illuminated by it show 

 some colour. 



In order to remedy the defects of chromatism and want of 

 aperture, the following arrangement was completed and tried (see 

 plate XIV. fig. 1). A is an ordinary achromatic lens, of an inch 

 focus. B is a square prism of dense flint, with a polished surface, 

 C, making an angle of 67|- degrees to the base. At D is a double 

 convex lens of crown glass, of a quarter of an inch radius, cemented 

 into a corresponding cavity of the same curvature, worked in the 

 base of the prism, by which means it is made achromatic. 



At the apex of the prism is another double convex lens of crown 

 glass, E, with the contact surface of ^th of an inch radius, cemented 

 in the same way as at D. The total curves of the glasses are 

 arranged so as finally to produce achromatic light. 



The rays of light, after passing through the glasses A and D, are 

 reflected by the polished surface, and then farther condensed by the 

 lens E, placed at right angles to the axis of the emergent rays. 

 This arrangement has a similar effect to three achromatics, shown 

 by the dotted lines (which represent the concaves by that portion of 

 the prism they cut off), in combination with a reflecting surface. 



By means of this appliance a pencil of light was thrown obliquely 

 at an angle of forty-five degrees across the object, an aperture of 

 sixty degrees was obtained, and by the combination the markings on 

 the Navicula angulata have been shown with a quarter-inch object- 



