82 THE USE OF THE MICROSCOPE 



corrected condensers since for ordinary work; one, at least, 

 of which seems to have been calculated by Abbe himself.) 

 This uncorrected condenser was no doubt an improvement 

 on the concave mirror. Doubtless, Abbe's name attached 

 to it has caused this condenser to be more employed. 

 It was originally meant for use with a large source of distant 

 light, namely a white cloud. Its whole aperture was not 

 intended to be utilized, but only a direct or oblique cone of 

 light of about 0.3 aperture. The possibility of obtaining 

 cones of greater obliquity than could be given by the con- 

 cave mirror was its chief advantage. It was made to be 

 used as a water-immersion condenser, when employed 

 with oblique beams with high powers; but it is now fitted 

 on most routine microscopes, for a direct, not an oblique 

 beam, and to be used dry. In biological work the use of 

 oblique beams is by no means an everyday procedure; 

 nor is it to be recommended, having been replaced either 

 by full cones or by dark-field condensers. Objectives 

 have been improved since 1873, and so have condensers. 

 Hence it is, in the writer's opinion, unwise to retain, on 

 high-power microscopes, the uncorrected condenser, and 

 the rackwork for oblique illumination. (Allowance must, 

 of course, be made for the conservativism of the public 

 and the manufacturer.) 



Koch used the uncorrected condenser with full aperture 

 to veil the reflections and refractions in the object by the 

 resulting glare; thus leaving the colors of the well-stained 

 bacteria conspicuous on the luminous background. This 

 method may, of course, be employed with any condenser 

 of large aperture, for in this way, the parts of the object 

 visible by differences of reflection or diffraction are illu- 

 minated by Ught outside the aperture of the objective, 

 as if on a dark field. So they are whitened and disappear in 

 the Hght fog, leaving the color of the interior alone con- 

 spicuous. This phenomenon is visible, as is well known, 

 in a low-power objective of, for example, 0.3 aperture, 

 if a larger aperture than 0.3 is used in the condenser; 

 or if, with a condenser aperture somewhat less than 0.3, 



