26 The Microscope 



Oculars. Since the substage condenser permits the objectives to work 

 at relatively high numerical apertures, a Xl5 ocular is a useful adjunct 

 to the standard XlO. 



Substage Condenser. An ordinary Abbe condenser, with a maximum 

 numerical aperture of about 0.9, is all that is wanted. It will, of course, 

 have a built-in iris diaphragm but neither the condenser nor the diaphragm 

 should be in centerable mounts. These mounts require constant adjust- 

 ments, which are necessary only when working with apochromatic ob- 

 jectives at the limit of their resolution. 



Illuminating System. All this equipment is relatively worthless unless 

 it is accompanied by a proper lamp. Those doubting this point should 

 reread Chapter 1. A proper lamp is one having a small intense illuminant, 

 a focusable condensing lens, a field iris in front of the lens and a filter 

 holder in front of the field iris. The intensity of illumination should be 

 controllable, either through the provision of a rheostat or, in the writer's 

 opinion better, through the purchase of a set of neutral density filters. 

 The cost of this lamp should be carefully balanced against the increased 

 cost of purchasing a microscope with an illuminating system built into 

 the stand. Illuminating systems built into the substage are not very satis- 

 factory, but those built into the stand permit the erection of Kohler illu- 

 mination (described below) almost without effort. Until very recently 

 these stands were in the research class, but there are signs that they may 

 soon become available in a price range to compete with the combined 

 cost of a medical microscope and lamp. 



The equipment described above is perfectly adequate for teaching 

 histology, cytology, bacteriology, and any medical school course. It is 

 of little value to elementary courses because of the difficulty involved in 

 setting it up. 



Setting Up the Medical Microscope. A few words will have to be said 

 on the practice of microscopy before giving a step-by-step description. 

 Lenses are theoretically designed to work at their best when they are 

 examining a self-luminous object. This theoretical ideal is closely ap- 

 proximated if an image of a self-luminous object is superimposed on the 

 transparent object actually being examined. This was actually possible 

 in the early days when microscopists used oil lamps with broad wicks, 

 and the original function of the substage condenser was to cast an image 

 of this big flame into the field of the object. Some lamps had wicks as 

 broad as 4 in. so that the image of the flame would fill the field of view 

 of the objective. Images of the spirally coiled filaments of incandescent 

 lamps are not, to put it mildly, so satisfactory. 



The same optical conditions are met if the light radiating from a 

 point source is gathered by a well-corrected lens and if the surface of 

 the lens is used as the luminous source. There was a period in micros- 



