336 SCIENTIFIC APPARATUS. 



angle of divergence of the light from the condenser should be 

 nearly equal to, but not greater than, that which can pass into 

 the object-glass, or, in other words, that the angle of aperture of 

 both sets of lenses should be nearly equal, though at the same 

 time there should be the power of modifying this angle by means 

 of a rotating diaphragm with stops of various character. With 

 low powers, a diaphragm of larger size is used under the stage 

 without the condenser. In both cases an aperture that is abso- 

 lutely necessary to show certain objects distinctly makes others 

 quite invisible. Much the. same may be said respecting the value 

 of a so-called iris diaphragm, J t>r of one with holes of various sizes 

 fixed under the stage, used without the condenser to limit the 

 width of the beam of light thrown on the object. 



For certain cases it is useful to employ light, so transmitted 

 obliquely from below the stage to the under side of the object 

 that the field of the microscope is dark when the object is 

 removed. With such illumination, the structure of the object is 

 shown by light irregularly refracted, or so reflected over and over 

 again that its course is bent from the original oblique angle into 

 that of the axis of the instrument. This kind of illumination by 

 refraction may be obtained by throwing the light obliquely from 

 the ordinary mirror, or by bending it with a prism, or by means of 

 a lens or parabolic reflector with central stops. The latter two 

 give the effect of illumination from all sides, whereas the others 

 give that of illumination from only one side. Each have their 

 own special advantages. 



For many purposes no light is equal to that reflected from a 

 white cloud ; but since this cannot always be obtained, recourse 

 must be had to lamps of various kinds. The chief requisite in 

 these is to give a bright, white, and steady flame of moderate 

 size. Parallel rays may be advantageously thrown on the mirror 

 of the microscope by means of a bull's-eye condenser, placed so 

 that the flame is nearly in the focus. 



