DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 133 



tirely new field to the geologist, and one which promises 

 some very important results in throwing light upon the 

 original formation of that stone. Some beds of flints are 

 crowded with these infusoria, whilst others are wholly 

 without them. In viewing these creatures (crystatella) 

 a powerful object-glass is requisite ; and, as their 

 multi-pronged spines protrude from all parts of the 

 surface of their bodies, each part being in a different 

 plane, there must be a new adjustment of the focus. 



Some of the cavities in minerals contain (t two fluids 

 unknown to the chemist : groups of crystals, floating 

 bulls, &c." The forms of these cavities are various. 

 Those containing two fluids, which do not mix though 

 in the same cavity, appear globular. 



SECT. 13. Concave Specula, Silver Cups, or Leiberkuhns, 

 for illuminating opaque objects. In viewing opaque 

 bodies by means of concave specula, it is generally 

 advisable to remove the stage of the instrument, and to 

 insert the arm, fig. 16, with the forceps, fig. 15. An 

 object-glass of a right focus must then be screwed into 

 the body of the microscope, and the silver cup, properly 

 fitted, made to slide over it. If candle-light, which is 

 the simplest artificial illumination, be used, although a 

 small argand lamp is often preferable, the light may be 

 placed from about 3 to 6 inches behind the object. 



When we have day-light, the concave mirror must 

 be brought into the rear of the object. In this case it 



