The Microscope. 313 



demand of the present time. This demand has developed the large, 

 wide- angled condenser of Abbe, and this accessory has become indis- 

 pensible to advanced histological research, and with which may be 

 accomplished almost all the various illuminating effects. The field 

 of light it gives is so soft, even and controllable, that it must in the 

 future replace all other sub-stage illuminators. 



The apparatus consists of two non- achromatic lenses, mounted 

 in a short tube. The upper lens is plano-convex, and is greater than 

 a hemisphere; when properly adjusted, its plane surface lies slightly 

 below the upper plane of the stage. The focus of the combination 

 is only a few millimeters above the plane-surface of the upper lens, 

 and is therefore very near to the object. The angle of aperture of 

 the emergent rays is 120° in water, so that the marginal rays are 

 inclined to the axis at an angle of nearly 60° in water. The 

 extremely large size of the condenser admits all the light that can 

 pass through the sub-stage. With such a combination, the light at 

 the focal point is very intense — in fact, it is so great that all of it caa 

 be used only in certain examinations. Below the lenses is placed a 

 ring adjusted by different mechanical devices by the various manu- 

 facturers, which, in its recess, receives diaphragms with various sized 

 apertures. A diaphragm with central stop for dark-ground illumi- 

 nation, and one with an eccentric aperture for giving various grades 

 of oblique light, accompany the apparatus. 



The plane-surface of the upper lens is intended to be in homo- 

 geneous, immersive contact with the lower surface of the slide, but 

 as the effect of this contact is merely to slightly increase the intens- 

 ity of the light, which is often too great, it may generally be used 

 dry. 



To use the apparatus for ordinary histological examinations, the 

 condenser should at first be placed close under the slide, and the 

 light thrown into it from the -plane surface of the mirror. Then, 

 according to the intensity of the light required, the condenser may 

 be racked down, and by experiment the diaphragm with the sized 

 aperture to give the best definition. Experience only will give the 

 operator skill in this adjustment. 



For the examination of stained bacteria, the condenser should 

 be left open. As we mentioned in a recent issue, the effect of the 

 intense light thus given is to bring out in bold relief the stained 

 bacteria, and make invisible the detail of the surrounding structure. 

 As Koch terms it, it "isolates the stained image." 



