206 MICROSCOPIC ILLUSTRATIONS. 



this condenser is used, the flame of an Argand lamp 

 should be employed, shaded, however, by an external 

 copper tube, with an aperture in its inferior part just 

 large enough to expose the flame, placed over the usual 

 glass one. This will give increased effect and splendour 

 to the object, by preserving the apartment in gloom, and 

 thus allowing the iris to expand itself. 



I do not think a more intense light is got with an 

 Argand lamp than with a rushlight, but certainly a 

 far greater quantity of it. 



Silver Cups, or Specula, afford a very brilliant and 

 intense light, almost ivithout shadow, because it plays 

 vertically upon the summit of an object, like the sun of 

 tropical climates. This is the only species of illumination 

 which will bring out many opaque objects properly (a 

 fly's foot, or human hair, for example) ; but for others, 

 requiring shade for their verification, it is altogether 

 improper. (The markings on the scales of butterflies, 

 &c. are a good illustration of this position.) 



Each object-glass must have a cup attached to it ca- 

 pable of adjustment, by being moved up and down upon 

 the tube in which the object-glass is set, so as to cause the 

 focus of the latter to coincide with that of the cup, (the 

 maximum of brightness cannot otherwise be easily 

 attained) . The object must alivays be held by the nippers 

 or mounted on a cork cylinder, when it is to be illuminated 

 by cups : when the nippers only are employed, the disk, 

 7, must be placed between them and the illuminating 



