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glass, though not satisfactorily, for on attentively observing the 

 effects produced, it is apparent that the principle of transmitting 

 oblique light through a transparent object, in one direction only, is 

 faulty, as it appears on some objects (the Podura for instance) to 

 elongate the markings, if either projections or depressions, and 

 gives to dots or concavities the semblance of lines, producing a kind 

 of overlapping shadow (at some distance above the object), upon 

 which the markings are seen ; the light also being in some degree 

 decomposed by the margin of the object, the outline is rendered 

 indistinct; which circumstances are sufficient to show that there is a 

 defect in the principle of illumination from one side only. 



This appearance of a shadow suspended over the object, in a 

 position opposite to the direction of the light, would lead us to 

 conclude that, in order to balance this defect, the oblique rays 

 should be counteracted by a pencil of light, of similar form and 

 intensity, thrown on the object, at the same angle, from an opposite 

 direction. This it is proposed to effect, as shown by fig. 2, by 

 means of two of Nachet's prisms, placed in juxtaposition, and made 

 to revolve under the stage of the microscope, for the purpose of 

 placing the two pencils of light in various directions across the 

 object. 



The prisms should either have achromatic glasses, cemented on 

 at AA, or be made of flint, with double convex lenses of crown 

 glass, cemented in as before mentioned. Between the two prisms 

 at B there should either be a dark well or a black ground, and at 

 the base there should be a square diaphragm, CC. Although this 

 arrangement has not yet been tried, there is some reason to suppose 

 it would answer well. 



The cause of the markings on test-objects being seen by oblique 

 illumination, which cannot be shown by the ordinary method, 

 appears to be that in the former case no greater amount of light is 

 allowed to pass through the object into the object-glass than is 

 sufficient to render the structure distinctly visible, consequently 

 the object is not drowned by an intense glare of light, which is 

 sometimes occasioned by ordinary direct illumination. From this 

 it would appear that no rays from the source of light should enter 

 the object-glass by passing through or around the object, which 

 must be illuminated by very intense light, thrown on in all or 

 opposite directions, at an angle exceeding the aperture of the object- 

 glass, so that the light which enters the microscope should be that 

 wliich radiates only from the object, as if it were self-luminous. 



