FORMS OF LAMPS 159 



Illumination 



With regard to illumination, daylight from a north 

 window may be used for low powers, but even for these 

 is frequently insufficient in this country. Failing this, and 

 for all high-power work, some artificial illuminant must 

 be employed. An ideal illuminant is hard to find it 

 must be steady, as structureless as possible, and the 

 source should be quite small in area. The " Pointolite " 

 electric lamp of the Ediswan Co. probably fulfils these 

 conditions more nearly than any other form of lamp. In 

 it a small ball of tungsten is rendered incandescent it is 

 bright enough for all purposes, is steady, and the source 

 of light is small and structureless. Small electric arc 

 lamps are suitable only for photomicrographic work. 



For general use metallic filament electric lamps 

 are convenient, but the bulbs must be frosted. 

 They have the disadvantage that the source of light- 

 is a very extended one, particularly if considerable illu- 

 mination is required. A form by Steam, the "Zigzag," 

 is the most suitable. The Nernst electric lamp enclosed 

 in a shade (Barnard pattern) was very good, and will pro- 

 bably be soon procurable once more. If gas only is avail- 

 able, the incandescent mantle lamp may be used. The 

 grain of the mantle is, however, visible when the con- 

 denser is in use and focussed ; a fine ground-glass screen 

 may be interposed or, for this and also for filament 

 electric lamps, a round clear-glass flask filled with water 

 tinged with ammoniacal copper solution may be inter- 

 posed. This device annuls the structure of mantle or 

 filament and gives a beautifully soft light. 



Barnard has devised a gas mantle lamp in which the 

 mantle is made into a roll, and the end of the roll pro- 

 jecting from a tube-holder is rendered incandescent in a 

 Bunsen flame. 



