172 PKACTICAL PHOTO-MICROGKAPHY 



light, so that a parallel beam is projected. The lens No. II 

 is then placed in such a position that it takes up this parallel 

 beam and focusses a rough image of the source of light on to 

 the iris- diaphragm of the sub-stage condenser of the microscope. 

 In this case the auxiliary iris- diaphragm occupies a position 

 between the two condensing-lenses, and is therefore controlling 

 the diameter of the parallel beam. The image of the radiant 

 as seen at the back of the sub-stage of the microscope is also 

 much smaller ; but, as with medium- power objectives the 

 aperture of the sub-stage condenser is of necessity less, or at 

 least its effective working aperture is less, the projected image 

 is sufficiently large to fill the field and does so without any 

 reduction of aperture. In this case again the image of the closed 

 auxiliary iris may be focussed in the field of view. 



Eeference to Fig. 21 c will show the arrangement for low 

 powers. It does not differ in principle from the preceding 

 arrangement, except that the lens No. Ill, of shorter focus than 

 No. II, is substituted. The lens No. Ill is in fact similar 

 to No. I, but, instead of having its plane side towards the 

 illuminant, is placed with its concave side in that direction, 

 so that it throws a small image of the source of light on to the 

 sub-stage iris- diaphragm. The same conditions are here again 

 to be complied with as in the preceding example, and the image 

 of the auxiliary iris is once more focussed in the field of view. 

 It is assumed in the latter case that the sub-stage condenser 

 is one of large aperture, and probably will be one that is used 

 for higher power work, but has had its top combination removed. 

 In this case it will be found that the working aperture of the 

 condenser is, of course, again reduced ; but this is not a matter 

 of moment, as the objectives to be used in conjunction with it 

 "are themselves of proportionately smaller aperture. These 

 arrangements are to some extent influenced by the illuminant 

 in use. For instance, the combination of Nos. I and III 

 condensers, if used in conjunction with a Welsbach inverted 

 incandescent light, will answer perfectly for medium as well as 

 for low powers, so that the conditions may be altered somewhat 

 according to the size of the illuminant that is in use. Again, 

 the arrangement of the auxiliary condensers is to some extent 

 dependent on the sub-stage condenser, as on its focal length 

 depends to a great extent the size of the image of the illuminant 



