Miscellaneous. 261 



On a new Method of Illuminating Opake Objects, for the high 

 powers of the Microscope ; and on a new Achromatic Condenser. 

 By J. L. RiDDELi,. 



The front or terminal combination of the objective is made to con- 

 dense light upon the opake object, by sending rays of light from 

 behind, through the marginal border of the lens. 



To accomplish this, a circular disk of fine plate glass, say near a 

 fourth or fifth part as thick as the diameter of the lens, is bevelled 

 on its outer margin, by grinding and polishing to an angle of 45°. 

 A hole is drilled through the centre of the disk, of a diameter, say 

 two-thirds, three-fourths or four-fifths (dependent upon the angle of 

 aperture), as great as that of the lens. The margin of this hole is 

 also bevelled at an angle of 45°, down to a clean sharp edge. Both 

 rings of bevels are on the same side of the glass, so that if considered 

 as projected, the lines would cross each other at right angles. 



I find no insurmountable difficulty in giving an exquisite form and 

 finish to these disks. I mount and revolve the disk on a good rose 

 lathe ; at the same time the grinding or pohshing tool is revolved by 

 an overhead motion, the spindle carrying the tool being mounted 

 upon a slide-rest, and admitting of a protrusive movement at an 

 angle of 45° to the axis of the lathe. 



The disk, being finished, is to be placed centrally behind the 

 lens, the bevelled margins looking backward, and the sharp inner 

 edge almost or quite touching the lens. Parallel rays of light being 

 thrown upon the disk, in the direction of the axis of the objective, 

 from below in the direct, from above in the inverted microscope, 

 a ring of parallel rays is sent, by two successive internal reflections 

 from the bevelled surfaces, so that, with direction reversed, the light 

 traverses the outer margin of the objective, and by it is condensed 

 upon the object in focus. 



I tested this method of illumination in March last, sufficiently to 

 be satisfied of its great value ; more especially where the objective is 

 of very short focal distance, and where consequently other means of 

 illuminating opake objects cannot, on account of the nearness of the 

 objective to the object, be resorted to. 



New kind of Achromatic Condenser suggested. 



A larger, thicker, similarly bevelled disk, with the bevels on oppo- 

 site sides of the plate glass, and their lines of inclination coincident, 

 would probably serve as an efficient achromatic condenser of parallel 

 rays. By attaching centrally, on the side opposite the bevel, achro- 

 matic lenses of proper size, or a good doublet combination, a most 

 valuable form of achromatic condenser would I think be produced, 

 useful for general microscopic illumination. I have not yet put the 

 plan in practice. — Sillhnan's American Journal, January 1853. 



University of La., New Orleans, Oct. 4, 1852. 



STRUCTURE OF THE CELLS OF PLANTS. 



Physiologists are at the present day almost imanimous in their 

 notions of the normal structure of the cells of plants. An outer 



