ILLUMINATION 71 



its image on the object equal to (or less than) the object- 

 field of the particular combination of objective and eye- 

 piece employed, should not be omitted; as it improves 

 vision decidedly, especially with dry and water-immersion 

 objectives, allowing a higher aperture to be used; and gives 

 better contrast in photography. (An iris diaphragm before 

 the ground glass sometimes gets too hot, and sticks.) 

 The use of any sample of ground glass which can be shown 

 to let through some direct light (that is, not diffused or 

 diffracted) causes a loss of light; since this direct light 

 is not concentrated at the same focus as the light diffracted 

 by the ground glass itself, and helps to cause fogging 

 reflections. In the writer's experience, properly ground 

 glass before the lamp gives the best illumination available 

 at present for general practical work with the microscope. 

 It allows of the diaphragm being close to the (secondary) 

 source. If the grain of the matt surface is fine, it is not 

 noticeable with the high powers; and with the low powers 

 it may be thrown slightly out of focus (though this is 

 seldom necessary) . 



Use of a Bull's-eye Condensing Lens. — Nothing need 

 be said about an uncorrected bull's eye, except that its 

 use is rationally to be confined to an uncorrected condenser. 

 There are three methods of using a corrected bull's eye. 

 In the first method, the small light source may be magnified 

 by a corrected bull's-eye lens, less than its focal length 

 from the source. This method was recommended by 

 Coles for dark field; he, however, used an uncorrected 

 bull's eye, introducing spherical and chromatic aberrations. 

 Apparently no corrected condensing lens had been cal- 

 culated for this purpose. A special aspheric bull's eye 

 has been figured for this, by Bausch and Lomb, with which 

 an aspheric condenser should be used, and a yellow-green 

 screen. The diaphragms must be put close in front of the 

 source; and the condenser focuses these diaphragms on 

 the slide, where there is thus a magnified image of tlie 

 source. If the bull's eye is moved aside, the lamp may be 

 used directly for the high powers, if set farther back. 



