206 DR. HALL, ON AN EASY METHOD 



and we almost envy the sensation of delight which must be 

 experienced on using this simple means of illumination, and 

 beholding for the first time the wonderful markings and 

 brilliant colouring of many of these dust-like atoms, found 

 alike in the clearest waters, in the strongly acidulated, and in 

 the salt fluids of the various zones of the earth. " In springs, 

 rivers, lakes, and seas, in the internal moisture of living 

 plants and animal bodies, exists," says Pritchett, " a world, 

 by the common senses of mankind unperceived ; ij^, in the 

 ordinary pursuits of life, this mysterious and infinite kingdom 

 of living creatures is passed by without knowledge of, or 

 interest in, its wonders." To facilitate the investigation 

 of these wonderful organizations the present paper has 

 been written. 



The discovery of this method of exhibiting the Pleurosigma 

 angulatum was perfectly accidental, and so far as I know it 

 has not been published ; at any rate, Messrs. Powell and 

 Lealand were wholly unaware that such an effect could be 

 produced, on my communicating it to them. 



With man]/ microscopes is furnished, for the purpose of a 

 dark-ground illumination, what is called a " spotted lens ; " by 

 this means the object itself appears beautifully illuminated, 

 while the entire field by which it is surrounded is perfectly 

 dark ; the effect is produced by preventing any rays of light, 

 reflected from the mirror, passing through the object : this is 

 accomplished by placing a dark stop beneath the latter. The 

 arrangement, however, is such, that any oblique rays will 

 impinge upon the object, and after they are refracted by it, 

 they will pass into the object-glass ; consequently, the result 

 being, that the only rays transmitted through the instru- 

 ment, are those thus refracted from the object, it appears 

 beautifully bright whilst the surrounding field is black. The 

 accompanying engraving explains the instrument used by 

 myself, the cost of which was only Is. Qd. : a, the brass 

 tube, fitting either into the usual brasswork of the achro- 

 matic condenser of Smith and Beck, by which it can easily 

 be moved up and down and correctly adjusted ; or, as in the 

 microscopes of Mr. Salmon, into a small piece of tube 

 adapted to the diaphragm ; b b, the lens removed from the 

 tube. The drawings are the exact size of the different parts 

 of the apparatus. 



This instrument is usually employed with the lower powers 

 {the inch and tivo inches), when the appearance is that already 

 described. On using it with a quarter constructed for me by 

 Mr. Ross, the angular aperture of which is 85°, with a l-5th 

 of Smith and Beck's (angular aperture of 100°), and with a 



