MEMORANDA. 



303 



and tends to show analogically the very great increase of 

 distinctness that would be obtained if the object could be 

 seen in the same medium with the full aperture of the object- 

 glass. Having been rather curious to know if objects in 

 balsam could be observed under such an advantage, I have 

 tried a few experiments, which were successful in their 

 results. 



I first took a small hemispherical lens, of about l-90th of an 

 inch radius, and cemented it over a selected specimen of one 

 of the Diatomacea^ (iV. sigmd) with Canada balsam, in the 

 manner represented by the annexed diagram. A, the object.; 



h b, slide upon whicli it is mounted ; c c, hemisphere of glass 

 covering the object ; d d, pencils of rays diverging from the 

 object — these may also be considered to represent the aperture 

 of the object-glass. It will be seen from the position of the 

 object, tliat each ray of light passing from that point through 

 the surface of the hemisphere, will be transmitted in straight 

 lines, in a radial direction, without undergoing any refrac- 

 tion ; the consequence of which is, that the full and undi- 

 minished aperture of the object-glass is made to bear upon 

 the object. 



If an object is already covered with thin glass, it may be 

 surmounted with a lens, so far short of a hemisphere as the 

 thickness of the cover, which of course amounts to the same 

 in effect as if the lens were hemispherical. I have a specimen 

 of P. formosum^ mounted in this manner, by which the 

 markings are remarkably well displayed. A more simple 

 method of obtaining a similar result is by the following 

 course of proceeding: — Spread some of the desired forms of 

 Diatomacece upon a glass slip while in a moist state, and when 

 dry, scatter a number of small fragments of hard Canada 

 balsam upon the same surface ; apply heat very qradually, and 

 these will each run into the form of a spherule ; they will 



