Apertures of Object-glasses. 31 



represented by the annexed diagram It will be seen from 



the position of the object, that 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 

 refraction, the consequence of which is that the full and un- 

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

 object. 



" If an object is already covered with thin glass it may be sur^ 

 mounted 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 Diatomaceee upon a slip of glass 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 gradually 

 and these will run into the form of a spherule ; they will next 

 slowly sink into a shape approaching to that of a hemisphere; 

 Before the figure is quite completed, place the slide under the 

 microscope, and ascertain if any one of the nodules of balsam 

 exactly covers a fair specimen ; if not, the trial must be repeated 

 with a fresh slide. Having found an object properly situated 

 under the particle of balsam, the next step is to bring the latter 

 down to the form of a hemisphere, by the further aid of heat very 

 cautiously applied. The nodule of balsam having been too spherical 

 in the first instance, will now gradually sink, and must be repeatedly 

 tested under the microscope, till the perfect hemisphere is obtained 

 without any refraction being produced on the rays from the object 

 in the centre. The criteria for knowing this are: — first, the object 

 under the balsam must be in the same 'plane of focus as similar dry 

 objects outside ; secondly, the balsam object must not appear more 

 magnified than its uncovered fellows ; and thirdly, the balsam - 

 covered object should not require a different adjustment from the 

 dry ones on the same slide. The existence of these combined condi- 

 tions indicates a perfect hemisphere. 



'• When an object is seen under these circumstances it at once 

 shows the great increase of distinctness that is to be obtained in the 

 structure of the more difficult diatomaceous tests ; when they are 

 thus viewed in Canada balsam with the full aperture of the object- 

 glass, markings which in the neighbouring dry objects of the same 

 character are scarcely discernible, are sharply and distinctly visible 

 under the balsam hemisphere with the same illumination. 



" The luminosity of the field of view around the balsam object 

 is many shades darker than in the uncovered portion of the slide, 

 which appearance is caused by the diminished angle or cone of rays 



VOL. IX. D 



