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. 



