POWELL'S CHROMATIC, ABBE'S ACHROMATIC CONDENSER 3 1 [ 



pui-po.se of centring, and the movement is made by means of an 

 outer sliding tube 1>, with a slot at the top in which the arm A fits. 

 and another arm. B, is placed at the lower end so as to give ready 

 command of the rotation. This plan allows of the nse of one or t\vo 

 oblique pencils incident 90 apart in azimuth. The condenser thus 

 mounted is only intended as an oblique illuminator. It forms one 

 of the best of the very cheap condensers when it is mounted in a 

 plain tube mount with a ledge to hold the diaphragms. I) is the 

 optical part of the condenser placed immediately above the dia- 

 phragms and in oil-immersion contact with the base of the slide. 

 The circular diaphragm is fixed into the inner tube attached to the 

 sub -stage tube C, just below the position of the arm A ; the other 

 diaphragm is screwed to it by a screw in the eccentric hole, shown 

 in each. It will lie seen that when the diaphragms are placed 

 together in this manner the 

 movement of the arm will 

 produce the changes in the 

 light as above mentioned. 



As we intimated above. 

 Professor Abbe subsequently 

 produced an achromatic con- 

 denser, ostensibly for u>e in 

 high-power photographic 

 work, but in fact of much 

 more general utility. It 

 consisted of a single front 

 with two double backs, and 

 it projects a sharp and per 

 fectly achromatic image of 

 the source of light in the 

 plane of the object. Its 

 power is low, being ^ inch 

 focus, and it has a total 

 aperture of I'D. Its great 

 superiority over the chro- 

 matic form is that it trans- 

 mits a much larger aplanatic cone than that ; for whereas the former 

 gave only an aplanatic cone of '5, this instrument yields a similar 

 cone of - 65. But we have already expressed our pleasure that even 

 this form has been surpassed by the high quality condenser illustrated 

 in fig. 257. Like its predecessor, it is large and heavy ; and. with 

 great deference and respect to our Continental neighbours, we would 

 suggest that this is a too general characteristic ; the back lens in this 

 case is more than an inch in diameter, while barely | of an inch is 

 utilised when it is transmitting its largest cone. A very excellent 

 modification in fitting it to English microscopes has been made by 

 Mr. Charles Baker, the optician, which is shown in fig. 258. where 

 it will be seen that the fitting for stops is conveniently placed, and 

 an iris diaphragm can be used with great ease below this. This 

 ' turn-out : arm carries a disc of metal to receive the diaphragms, 



PIG. 256. Powell and Lealainl's 

 chromatic oil condenser (1880). 



