ACCESSORY INSTRUMENTS, 



113 



frame, is moved up or down by two small handles. For cor- 

 rect definition, Dr. "WoUaston employed a stop immediately 

 above the mirror, between the mirror and the lens, but 

 it has been found much better in practice to apply the stop 

 between the lens and the object ; this improvement was made 

 by Dr. Goring, and by it the length of tube employed is not 

 only much shorter than that suggested by WoUaston, but the 

 definition is greatly improved by the arrangement. Dr. 

 WoUaston states that " the intensity of illumination wiU de- 

 pend upon the diameter of the Uluminating lens and the pro- 

 portion of the image to the perforation, and may be regulated 

 according to the wish of the observer." 



Achromatic Condenser.— The condenser of WoUaston, just 

 described, although a very great improvement over the ordi- 

 nary methods of illuminating, is, nevertheless, to a certain 

 extent, faulty, in consequence of not being suppUed with an 

 achromatic lens ; to remedy this inconvenience, M. Dujardin, 

 in 1840, contrived an instrument which he termed an eclair age, 

 for the purpose of illuminating objects with achromatic light ; 

 a modification of this apparatus is now supplied with aU the 

 best microscopes, and is known as the achromatic condenser, 

 and although it is applied in different ways to the micro- 

 scopes of our three eminent makers, it, nevertheless, consists of 



three essential parts ; viz., an 

 ^ achromatic combination, an 



_i adjustment of focus for the 



same, and a means of making 

 the axes of the object-glass 

 and of the condenser coincide 

 exactly. When the com- 

 pound body is made to turn 

 away from the stage, the 

 apparatus for adjusting the 

 axes is very simple, and the 

 plan adopted by Mr. Ross and 

 Mr. PoweU is represented by 

 fig. 58 ; it consists of two 

 tubes, sliding one within the 



Fig- 58. 



