AGf~ICULTURAL RESEARCH, PUSA.
AGf~ICULTURAL RESEARCH, PUSA.
AGf~ICULTURAL RESEARCH, PUSA.
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60 PRACTICAL BACTERIOLOGY<br />
be unreliable. The pointolite lamp requires a special<br />
resistance, and the best results are secured with the<br />
direct-current model, although a type for alternating<br />
current may be obtained; in addition, an enclosed<br />
chamber with a condensing lens has also to be purchased.<br />
Its light is not so intense as that of the arc,<br />
but once adjusted this lamp requires no further attention,<br />
and gives a constant light for any length of time.<br />
On the whole, the pointolite is perhaps the more<br />
cOllvenient illuminant when direct current is available.<br />
In view of the fact that alternating current is becoming<br />
almost universal, the use of special small gas-filled lamps<br />
having a short, thick and tightly wound spiral filament should<br />
be noted. 'l'he source of illumination is small, compact and<br />
intense. Previous filament lamps had a long, loosely wonnd,<br />
thin filamelre, and a satisfactory beam of light could not be<br />
obtained. The newer lamps take 5 amperes at 6 volts, and<br />
can be worked from any alternating-current supply through a<br />
suitable transformer. As with the arc and pointolite lamps,<br />
a ventilated enclosed chamber with a condensing lens is<br />
necessary, and complete lamps at small cost can now be<br />
obtainecl.<br />
The P'unnel Stop.-Whell the objectives employed for<br />
dark-ground illumination have a numerical aperture of<br />
more than 1·0 (as in the case of ordinary oil-immersion<br />
lenses), a special stop to reduce the N.A. to less than<br />
1·0 must be employed. This consists of a small funnelshaped<br />
piece of vulcanite which fits into the objective<br />
behind the back lens. It is advisable to procure the<br />
stop from the maker of the lens employed. The stop<br />
is removed by unscrewing, and the objective can at<br />
6nce be converted for ordinary use.<br />
Alternatively an objective adapter, with a small iris diaphragm,<br />
may be used. 'rhe front part of the oil-immersion<br />
objective is removed and screwed to the adapter, which then<br />
takes the place of the objective on the nose-piccc. The numerical<br />
aperture of the objective may be reduced as desired by<br />
manipulating the iris diaphragm in the adapter.<br />
Certain manufacturers have introduced for dark-ground<br />
illumination with bicentl'ic condensers, special oil-immersion<br />
fluorite objectives, which are used without a funnel stop.
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