304 MEMORANDA. 



next slowly sink into a shape approaching to that of a hemi- 

 sphere. Before this 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 hemi- 

 sphere, 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 re- 

 peatedly tested under the microscope till the perfect hemi- 

 sphere 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 magnified more 

 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 

 conditions indicate 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 tlie 

 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 of the illuminating pencil. From this 

 fact it is evident that the theoretical perfection of mounting 

 objects in this manner would be to enclose them exactly in 

 the centre of a minute sphere of balsam. In this case the 

 pencil of rays, both from the achromatic condenser and object- 

 glass, would pass directly to the object without refraction or 

 diminution ; but I must confess that I have not been success- 

 ful in effecting this. It is very easy to form a sphere of 

 balsam at the end of a needle-point of any required degree 

 of minuteness, but very difficult to coax an object into tlie 

 centre of such a spherule. — F. H. Wenham. 



iTiicroHcopicai Coiiver»azionc8. — Few events have excited more 

 interest amongst microscopical observers than two soirees, 

 given by Mr. N. B. Ward, as Master of the Apothecaries' 



