134 A MANUAL OF BACTERIOLOGY 



the full aperture of the diaphragm, or thereabouts, may 

 generally be employed. It must be remembered, however, 

 that the resolving power of a lens (see below) is diminished 

 by closing the diaphragm and by throwing the condenser 

 out of focus ; the illumination then becomes " non-critical." 

 For fine work, if the illumination is too intense, this should 

 be diminished by diminishing the source of light or by 

 interposing a coloured screen, such as Gifford's, which 

 consists of a cell containing a solution of malachite green 

 in which is inserted a piece of green signal glass. Coloured 

 glass may also be interposed. The microscopist should 

 accustom himself to examine specimens both by daylight 

 and by artificial light ; hanging-drop specimens are 

 usually best seen with the latter. For artificial light, 

 probably nothing surpasses a paraffin lamp with flat 

 wick, the edge of the flame being always used, while to 

 obtain the best results the mirror should be removed, and 

 the flame used direct by elevating and tilting the micro- 

 scope somewhat. For the finest work, daylight illumina- 

 tion is inadmissible. An admirable form of electric lamp 

 is the " Barnard," made by Messrs. Swift and Son, the 

 source of illumination being a Nernst lamp. For ordinary 

 routine work, an incandescent carbon or metal filament 

 electric lamp, a Nernst lamp, or an argand or incan- 

 descent gas burner may be used. Various devices have 

 been introduced for the employment of monochromatic 

 illumination, e.g. the quartz mercury vapour lamp by 

 Barnard. 



With the filament, Nernst, or incandescent gas, lamps, 

 the image of the filament or mantle is troublesome when 

 the condenser is in focus ; this may be obviated to some 

 extent by the use of frosted bulbs or by interposing a 

 screen of fine ground glass, by the use of Gordon's glass rod 

 illuminator, or by interposing a spherical flask filled with 

 water or dilute copper sulphate solution. Incandescent 



