CONDENSING LENS. 



121 



Fig. 69. 



reflected from it into the microscope ; and this mode of viewing them 

 may often be advantageously adopted in regard to semi-transparent 

 or even transparent objects, for the sake of the diverse aspects it 

 affords. Among the various methods devised for this purpose, the 

 one most generally adopted consists in the use of a Condensing Lens 

 (Fig. 69), either attached to the microscope, or mounted upon a 

 separate stand, 

 by which the 

 rays proceed- 

 ing from a lamp 

 or from a bright 

 sky are made to 

 converge upon 

 the object. For 

 the efficient il- 

 lumination of 

 large Opaque 

 objects, such as 

 Injected prepa- 

 rations, it is 

 desirable to em- 

 ploy a Bull's 

 Eye Condenser 

 (which is a 

 piano - convex 

 lens of short 

 focus, two or 

 three inches 

 in diameter), 

 mounted upon a 

 separate stand, 

 in such a man- 

 ner as to allow 

 of being placed 



Ordinary Condensing Lens. 



in a great variety of positions. The mounting shown in Fig. 70 

 is one of the best that can be adopted : the frame which carries the 

 lens is borne at the bottom upon a swivel-joint, which allows it to 

 be turned in any azimuth ; whilst it may be inclined at any angle 

 to the horizon, by the revolution of the horizontal tube to which it 

 is attached around the other horizontal tube which projects from 

 the stem ; by the sliding of one of these tubes within the other, 

 again, the horizontal arm may be lengthened or shortened ; the lens 

 may be secured in any position (as its weight is apt to drag it down 

 when it is inclined, unless the tubes be made to work, the one into 

 the other, more stiffly than is convenient) by means of a tightening 

 collar milled at its edges ; and finally the horizontal arm is attached 

 to a sprung socket, which slides up and down upon a vertical stem. 

 The optical effect of such a Lens differs according to the side of it 

 turned towards the light and the condition of the rays which fall 



