300 



ACCESSORY APPAEATUS 



good sis any other, because the diaphragm was placed at a distance 

 from the lens of at least five times its focus, so that the difference 

 between diaphragm focus and ' white cloud ' focus, or the focussing 

 of the image of a white cloud upon the object, was not very great. 

 But Brewster was writing of a flame from a saucer of burning spirit 

 and salt when he insisted on the bringing of the condenser to a 

 focus on the object, and in this he was, beyond all cavil, right. 



In 1839 Andrew Ross gave some rules for the illumination of 

 objects in the ' Penny Cyclopedia.' These were : 



1 . That the illuminating cone should equal the aperture of the 

 objective, and no more. 



2. With daylight, a white cloud being in focus, the object was 

 to be placed nearly at the apex of the cone. The object was seen 

 better sometimes above, and sometimes below, the apex of the cone. 



3. With lamplight a bull's-eye is to be used to parallelise the 

 rays, so that they may be similar to those coming from a white 

 cloud. 



Of the old forms of condenser, that devised by Mr. Gillett w r as, 



there can be no doubt, the 

 best. It was achromatic, and 

 had an aperture of 80. Fig. 

 239 illustrates it. It was 

 fitted with a rotating ring of 

 diaphragms placed close be- 

 hind the lens combination. 

 This was formed, as the figure 

 shows, by a conical ring with 

 apertures and stops. The 

 large number of apertures 

 and stops it would admit, 

 provided they are care- 

 fully 'centred,' are of great 

 value in practical w r ork ; 

 and the fact that they are 

 so placed as not to inter- 

 fere with the stage, makes 

 this arrangement of dia- 

 phragms and stops an excellent one, and it is not clear why it has 

 fallen into disuse. 



It had been the custom to recommend the use of this instrument 

 racked either within or without its focus.- Carpenter employed it 

 without, and Quekett within, and one or other of these methods was 

 general. But in the use of good achromatic condensers with high- 

 power work it soon became manifest to practical workers that it is 

 only when, as Sir David Brewster pointed out, the source of light is 

 focussed by the condenser on the object that a really critical image 

 is to be obtained. And Mr. Nelson readily demonstrated this fact 

 even with the condenser Gillett had devised. 



The next condenser of any moment is a most valuable one, and 

 constitutes one of the great modern improvements of the microscope. 

 It was an achromatic condenser of 1 70 devised and manufactured 



FIG. 239. Grillett's condenser, from 

 ' Hogg on the Microscope.' 



