68 THE USE OF THE MICROSCOPE 



diameter would be best. A diatom shows sharper reso- 

 lution when a small spot of light is focused on it, than when 

 the whole field is filled with (mostly unused) illumination. 

 This spot of light may be the direct image, cast by the 

 condenser, of a tungsarc or point-of-hght lamp; but this 

 may be far too bright, unless used with a dense screen. 

 Or it may be the diaphragmed image of a kerosene-lamp 

 flame, turned edgeways, or sideways, or with doubled 

 wick (Coles). Or it may be a short segment, properly 

 screened, of the incandescent coil of a C-Mazda electric 

 lamp; or the image of a 6-volt, coiled-filament or tungsten- 

 ribbon lamp, isolated by a diaphragm. Or a small circle 

 from a brightly illuminated double-ground plate of glass 

 may be used with effect, as Hartridge has shown (with 

 opal and frosted glass). But for all these there must be a 

 corrected condenser, if the optimum illumination is to be 

 attained. (The employment of an extra small source, 

 however, seems unnecessary on a dark ground, where the 

 image of the source on the slide should just coincide with 

 the object-field.) Beck (33) has shown that reflections 

 at cover-glass or slide surfaces cause extraneous light 

 (glare), which fogs the image, especially with dry or water- 

 immersion objectives. Such glare is more and more obvi- 

 ous the larger the source (Fig. 17). Hence it is evident 

 that the image of the source on the slide should not be 

 markedly larger than the object-field; that is, the source 

 should not exceed the source-field. Also, at every lens 

 surface there is 4 per cent or more of reflection, sometimes 

 causing glare. Such glare, with 20 surfaces, as in a high- 

 power binocular microscope, makes a large total. More 

 light, from a larger source, of course gives more glare. 

 Thus, a small circle of bright light in the object-field is the 

 ideal for optimum resolution and definition. For most 

 work, however, we must make the source about equal to the 

 source-field. 



In Nelson's original method (101) of using a lamp 

 flame, there may be three improvements: first, the flame, 

 or rather the most luminous part of it, may be diaphragmed 



