CH. II] DARK-GROUND ILLUMINATION WITH HIGH POWERS 766 



light for observation will be obtained. If the condenser is consid-' 

 erably too high or too low the object will be lighted with a ring of 

 light with a dark center. It is only when the hollow cone of light is 

 focused on the specimen that the entire field will be equally bright. 

 After a little practice one can focus the condenser on the specimen to 

 be studied . The image is so much more satisfactory that it is worth 

 the trouble. 



130. Getting the light to pass up through the condenser. The 

 microscope mirror should be about 20-30 cm. from the arc lamp or 

 from the io8-watt daylight lantern (fig. 5oc, d). It must be quite 

 close to the stereopticon lamp (fig. 500) . One can tell the best posi- 

 tion by holding a piece of white paper in the parallel beam to find the 

 position where the light is most uniform, and brightest. Place the 

 microscope so that the parallel beam strikes the mirror at this bright- 

 est point. The mirror will be nearly filled with the light. If it is 

 difficult to see on the mirror one can use some white paper over the 

 mirror. Sometimes the lamp-house must be tipped forward, or 

 the microscope raised or tipped backward, to bring the light beam 

 and mirror in proper relation. Now turn the mirror so that the light 

 will be turned up into the condenser. If the condenser has a drop 

 of cedar oil on it or if a slide is in immersion contact and a drop of 

 cedar oil is on the cover-glass one can see when the light passes through 

 the condenser. If now the oil immersion is focused on the object it 

 usually takes but a slight manipulation of the mirror to get the best 

 light. One should never be satisfied until the light is at its best, and 

 this can only be told by considerable experimenting. There is an- 

 other sign which gives great help, and is particularly applicable in 

 the evening or in a dimly lighted room. The light which passes from 

 the lamp to the mirror and then to the condenser passes on to the top 

 of the condenser where the lamp-filament is nearly at a focus. From 

 the top of the condenser there is always a considerable reflection and 

 this reflection passes back through the condenser to the mirror and 

 then back toward the lamp-house. Wherever it strikes will appear 

 a spot of light (fig. sob, e). If now the mirror is turned slightly until 

 this spot of light is thrown into the parallelizing lens the light will 

 also be very satisfactory on the specimen. After a little practice 

 one can light the microscope with great certainty by the aid of this 



