74 TEE USE OF THE MICROSCOPE 



put close to it, and, conyoquently, the regular condenser 

 aperture may be often increased, in the writer's experience, 

 to nine-tenths of that of the objective (Beck). Other 

 methods, employing corrected bull's-eye condensers in 

 addition to the usual condensers, require the apparatus 

 to be fixed to a board for accurate centering, and have other 

 inconveniences and sources of error. For the two-lens 

 uncorrected (so-called "Abbe") condenser, it is claimed 

 (Zeiss, Hartridge) that a good method is to put a disc 

 of ground glass in its diaphragm carrier, and illuminate 

 this strongly. 



Practical Points 



1. Focus the source of light on the object with a corrected and 

 well-adjusted condenser (Nelson). 



2. Put a diaphragm on the source of light so that the illumi- 

 nated circle on the slide only just fills the object-field, or is less 

 for the best vision (Beck). A 3-millimeter diaphragm may suit a 

 source 25 centimeters distant, for high powers. 



3. For low, medium and high powers, finely ground glass, close 

 to the lamp, is satisfactory. 



4. For the highest resolution, the kerosene-lamp flame or the 

 tungsten-ribbon lamp may be excellent; though they are too 

 small for low powers. 



5. The use of a bull's eye, even a corrected one, is trouble- 

 some; and may be a means of spoiling the corrections of the con- 

 denser, and lessening its aperture. 



6. For uncorrected condensers, it is regarded as easiest to form 

 an image of the source of light, by a bull's eye, close to the iris of 

 the condenser; or to adjust an illuminated ground glass there 

 (Hartridge, Zeiss). 



7. In all cases, some means of moderating the intensity of the 

 light is essential, since the iris of the condenser is to be used only 

 for regulating the aperture. 



8. When ground glass is used for low powers, a small thick 

 bull's eye (hemisphere) may be turned in between the ground 

 glass and the electric bulb, to give a small central disc of intense 

 light for the high powers. 



9. With a block of wood, a porcelain screw-down socket, 

 sheet copper, and a few screws, a lamp case can be made with 

 a movable front; allowing the centering of the pivoted diaphragms 

 with the brightest part of the ground glass. 



