85 



Those who have experimented on the subject may have observed 

 that there is something in the nature of oblique light, reflected from 

 a metallic surface, particularly favourable for the purpose of bringing 

 out minute markings, which may in some measure be attributed to 

 the circumstance of light so reflected being purely achromatic ; and 

 in order to avail myself of this property, I have designed a method 

 by which the condensation of lateral light may be effected by means 

 of a metallic reflector. 



The apparatus is drawn in fig. 3, the size of the original. AA is 

 a parabolic reflector, of a tenth of an inch focus, with a polished 

 silver surface, having the apex so far cut away as to bring the focal 

 point at such a distance above the top of the apparatus (which is 

 closed with a screw cap when not in use) as may allow the rays to 

 pass through the thickest glass commonly used for mounting objects 

 upon, before coming to a focus. At the base of the parabola is 

 placed a disk of thin glass, BB, in the centre of which is cemented 

 a dark well, with a flange equal in diameter to the aperture at the 

 top of the reflector, for the purpose of preventing the direct rays 

 from the source of light from passing through the apparatus. The 

 dark well is rather less than the diameter of the flange, and must be 

 raised as high as may be found necessary to prevent the light from 

 the reflector from entering the object-glass. 



The reflector is moved to or from the object by means of the 

 rack and pinion, C, and has similar adjustments for centering, and 

 is fixed under the stage of the microscope in the same way as the 

 ordinary achromatic condenser ; in addition, there is a revolving 

 diaphragm, D, made to slide on the bottom tube of the apparatus ; 

 it has two apertures, EE, placed diametrically, for the purpose of 

 obtaining two pencils of oblique light in opposite directions. 



The effects of the chromatic and spherical aberrations, in the 

 shape of fog and colour, about the objects, caused by the glass slides 

 upon which they are mounted, were sufficiently conspicuous, when 

 illuminated by the reflector just described, as to require a means of 

 compensation, for as the parabola has the property of throwing 

 parallel rays uncoloured to a point, when used alone it is most 

 suitable for an object without glass underneath it. 



Fig. 4 represents the aberrations of a large angular pencil of rays, 

 caused by a plate of glass. AA are rays reflected from the surface 

 of the parabola, proceeding in the direction of the focus, B, as shown 

 by the dotted lines. CC is an interposed plate of glass, which 



