372 OSCAR W. RICHARDS 



L. USEFUL ACCESSORY EQUIPMENT AND RECORDING 



METHODS 



1. Illumination and Its Importance 



No matter how excellent the microscope, detail can be seen only 

 when the illumination is equally excellent. What may be seen with 

 the microscope, true or false, depends on proper lighting. Daylight 

 is too uncertain for microscopy and has been replaced by electric 

 illuminators. These may be divided into two classes: (1) illumi- 

 nated surfaces and (2) lens systems for concentrating the light. With 

 the former, the lamp, mirror, and condenser of the microscope are 

 arranged so that both the field of view and the back lens of the objec- 

 tive, seen on removing the ocular, are uniformly tilled with light. 

 If the microscope has no condenser, one uses the concave mirror and 

 moves the lamp to the distance from the microscope that fills the 

 field with light. 



Lamps with an iris diaphragm and lens system are essential for the 

 best illumination of the microscope for comfortable vision and to fill 

 the aperture of the objective for full resolution without glare. With 

 the coiled-coil filament lamps {e.g., CC-13 filament), the lamp con- 

 denser is focused to give an image of the filament coils on the iris 

 of the microscope condenser; then the microscope condenser is ad- 

 justed to bring the diaphragm of the lamp condenser into focus on the 

 specimen and finally the microscope condenser aperture is opened 

 until the back lens of the objective is just filled with light. This 

 method of adjustment often is called Kohler's method. 



When the light source is large and sufficiently uniform to fill both 

 the field and the back aperture of the objective with light, the lamp 

 condenser may be focused to infinity (for practical purposes the fila- 

 ment is focused on a wall ten or so feet away) and the microscope 

 condenser adjusted to focus the filament on the specimen. This 

 method is called "critical" illumination, although it is actually no 

 more critical than Kohler's and other methods (10,47). This method 

 should be used for darkfi.eld and fluorescence microscopy and for op- 

 tical staining. 



Tungsten filament lamps are good light sources. The ribbon fila- 

 ment type gives the most uniform illumination. Since the entire 

 ribbon may not be imaged on the condenser and usually does not oper- 

 ate at as high a color temperature, it actually gives less light than the 

 coiled-coil type filaments. Of the latter, more of the CC-13 filament 



