24 



THE MICROSCOPE 



niumina- As regards the best kind of illumination for transparent 



transparent objects, the light may be a nearly parallel beam from the flat 

 objects. mirror, or a slightly divergent beam from the flat mirror used 

 with a lamp near the mirror. The light from a lamp may be 

 rendered nearly parallel by placing a bull's-eye condenser close 

 to the lamp. To find the correct position for the bull's-eye to 

 give parallel light, an image of the flame or filament of the lamp 

 should be observed on a distant wall and the bull's-eye moved till 

 the lamp or filament is in sharp focus on the wall. The light is 

 then approximately parallel, and the microscope should be so 

 placed that the mirror is in the beam of light about 8 or 10 inches 

 away from the lamp. 



The light may be made slightly convergent if the bull's-eye 

 be arranged to give parallel light, but the concave instead of the 

 flat mirror be made use of. 



The light may be condensed by a substage condenser, which 



not only increases the brilliancy of 

 the illumination, but also gives a 

 strongly convergent beam of light 

 which may be modified to any extent 

 by the use of the iris diaphragm and 

 stops, as described more fully under 

 the description of substage condensers. 

 The question as to whether the 

 best results will be obtained by 

 parallel, divergent, or convergent il- 

 lumination, depends to a great extent 

 on the nature of the object. When 

 light shines through certain kinds of 

 objects it is distributed or scattered 

 in all directions. A cut-glass lamp- 

 shade breaks up the light that falls 

 upon it and scatters it all round; the same thing occurs to 

 a lesser extent in the case of a botanical or histological section 

 of tissue when each cell or irregularity acts like a facet of a cut- 

 glass lamp-shade. In this case, whatever the nature of the 

 iUumination, there is a sufficiently scattered light to fill the 

 aperture of the object glass, and the general structure of the tissue 

 will be accurately depicted. This, however, does not apply to 

 all kinds of objects. Some do not scatter light, and the question 

 as to whether the aperture of the object glass is filled with light 

 depends on the nature of the illuminating beam. If an object 

 which does not scatter light is illuminated as shown in Fig. 12 {a), 

 the object glass might just as well have nothing but a pinhole 

 aperture ; and to make use of the aperture a convergent cone of 

 light must be thrown upon the object, as shown in Fig. 12 (6). 

 oft^Tbj ^^ ?^^^^ objects spread the light slightly by diflraction, 

 object. though in the case of a single dark object on a white field, the 



a 



Fig. 12. 



