162 
ELEMENTARY CHEMICAL MICROSCOPY 
light in dark-ground illumination or with vertical illuminators, 
but for oblique reflected light in the study of opaque objects, 
the size of the lamp bulb and the position of the tungsten fila¬ 
ment renders the lamp and condensers somewhat clumsy and 
apt to be in the way. To avoid eye fatigue', when using one 
of these powerful tungsten lamps, it should be screened or treated 
with frosting compound and graphite or aluminum. Two or 
three dippings will be required to produce a coating absolutely 
opaque. A window is then made by washing off a circular area 
with alcohol. 
As a general purpose lamp that shown in Fig. 95 leaves little 
to be desired. It consists of a concentrated filament, 6 volt, 
Fig. 95. Bausch & Lomb Optical Co. Adjustable Microscope Lamp. 
24 watt, tungsten lamp set in a cylindrical housing fitted with 
a powerful condensing lens. A small constant service step- 
down transformer is provided in order that it may be attached 
to a no volt A.C. lighting circuit. Although not quite power¬ 
ful enough for dark field illumination, fair results may never¬ 
theless be obtained; but for ordinary uses and especially for 
the illumination of opaque objects lying upon the stage this 
little lamp is the most satisfactory of those now on the American 
market. 
It is made with two types of condensing lenses — “ Spherical ” 
and “ Aspherical.” The latter is much the better. As listed 
by the maufacturers the vertical supporting rod is only 17 cm. 
high. It should be not less than 30 cm. for chemical micros¬ 
copy. The purchaser should therefore specify a 30 cm. stand. 
