300 



ACCESSORY APPARATUS 



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

 from the lens of .-it least five times its focus, so that the difference 

 lift ween diaphragm focus and white cloud ' focus, or the focussing 

 of the image <>f a white cloud upon the object, was not very 'great. 

 lint l!iv\vsTcr was writing of a name from a saucer of burning spirit 

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

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



In IS:!!) Andrew Ross gave some rules for the illumination of 

 objects in the ' Penny Cyclopaedia.' These were: 



!. 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 lie placed nearlv at the apex of the cone. The object was seen 

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

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

 ravs. so that they may lie similar to those coming from a white 

 cloud. 



( )f the old forms of condenser, that devised by Mr. C4illett was, 



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 work ; 

 and the fact that they are 

 so placed as not to inter- 

 fere with the stage, makes 

 this arrangement of dia- 

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



fallen into disuse. 



It had been thecnst to recommend the n.se of this instrument 



racked <-////o- ,i-illii,i in- tr'tlltaiit if.* foCliS. Carpenter employed it 

 u it hoi it , and (Bucket t within, and one or other of These met hods was 

 general. Put 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 Mrewster pointed out, the source <>i'/i<jhtis 



focussed /'// ///' condenser on the object that a really critical image 



i> to be obtained. And .Mr. Nelson readily del istrated this fact 



even with the condenser dillett had devised. 



I'll'' next condenser of any moment is a most valuable one. and 

 t it i lies one of t he great modern improvements of the microscope. 



I' was an achromatic condenser of !7<> c devised and manufactured 



I i'.. -Joli. (lillctt's condenser, from 

 on tin- Microscope.' 



