INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



525 



results, one being the more effective upon certain frustules, or with a 

 particular paraboloid, or a given objective. 



The diaphragm is a circle of the exact diameter of the tube in 

 which the paraboloid is mounted, and rests upon the ring in which the 

 lens is set, and is therefore in close contact with its base, excluding 

 all light except such as may enter through the aperture. It is sup- 

 ported by a slight inner tube sliding conveniently tight within that 

 holding the paraboloid. The stop fits the cup accurately, dips into it, 

 and completely lines its inner surface, admitting rays only perpen- 

 dicularly through the eccentric opening. The stop is thus substituted 

 for that usually supplied with a paraboloid (which has to be removed), 

 and a flat-headed pin passed through its central hole into the perfora- 

 tion in the paraboloid (fitting moderately tight), suffices to maintain 

 it steadily in position. No stop is needed with the diaphragm. 



Fig. 40. 



Fig. 41. 



Fig. 44. 



Fig. 43. 



Flc. 42. 



A slide of .4. pellucula being placed upon the stage, the paraboloid, 

 carrying its diaphragm and shutter, or stop, is inserted in tlio sub- 

 stage and racked up to focus, and the concave mirror manipuhited in 

 the axis of the Microscope until the light is thoroughly introduced 

 through the eccentric aiicrturo, tlio duo accomplishment of which 

 causes a brilliant but narrow linninous' streak to appear across the 

 slide. This arrangement cfTocted, objective focuR.«;ed, and valve found 

 (isolated, and ai)pcaring to be otherwise well placed), the st^igo is 

 rotated until the diatom under observation lies exactly lengthwise in 

 the direction of the beam of light; which is made evident by tlio 

 bluoish colour, and equally illuminated and undistortod form, of the 



