ILLUMINATING APPARATUS 



27 



to the large magmfication obtained with high-power lenses ; but 

 this is not the only advantage gained by the use of a condenser, 

 as illumination might be increased by other means — for instance, 

 by bringing a source of light closer to the object. A substage 

 condenser receiving an approximately parallel bundle of light 

 from the mirror of the microscope converts it into a wide angle 

 cone of light. When this light is centred and focussed, the object 

 is illuminated by light falling upon it in all directions. 



The achromatic condenser (Fig. 15) has the same aperture as Aohromatio 

 the Abbe condenser 1 N.A., but it is corrected almost as care- 

 fully as a microscope object 

 glass, so that the rays come to 

 exact points, and a very perfect 

 image of the source of illumina- 

 tion is formed in the plane of 

 the object, much reduced in size. 

 It is provided also with an iris 

 diaphragm and a tray for patch- 

 stops and filters. Fig. 16 shows 

 a beam of light (A, B, C, D, 

 E, F) passing through this con- 

 denser to the central poiijt of 

 the object at 0. The rays 

 A, F, which are at the margin 



of the beam of light as it enters, emerge as the most oblique 

 rays falling upon the object 0, and the rays C, D, which enter 

 near the centre of the condenser, emerge nearly parallel. Thus, if 

 the iris diaphragm which is placed below the condenser is gradu- 

 ally closed, it excludes more and more of the oblique rays. Fig. 16 

 shows a large solid cone of light of great angle converged upon the 

 object, the iris diaphragm being fully open. Fig. 17 shows a small- 

 angled cone transmitted by the same condenser, the iris diaphragm 

 being partially closed. Fig. 18 shows the same condenser in 

 which the iris diaphragm is fully open and 

 an opaque patch or stop is placed below 

 the condenser so that the object is being 



_ 



Fig. 



15. — ^No. 3288, Achromatic 

 Condenser. 



A B C D E F 

 Fio. 16. 



Fig. 17. 



Fig. 18. 



