188 METHODS OF MICROSCOPICAL EESEAECH. 



As to illumination, there is a great choice, but we 

 may at once dismiss for ordinary work : Sunlight, 

 as being too precarious, and necessitating mid-day 

 leisure ; Magnesium, as too expensive, and very 

 difficult to focus by, on account of its trick of going 

 out when left to itself. Electric arc necessitates the 

 charging and discharging of 30 or 40 Grove's cells, 

 a pleasure that can be appreciated only by those 

 who have tried it. In favour of Incandescent lamps 

 we cannot say a word. A 20-candle Swan lamp 

 requires as powerful a battery as a small arc, and 

 has none of its advantages. Its light is not con- 

 centrated, it is feeble and yellow. Lime light is the 

 cheapest, least troublesome, and, on the whole, the 

 best of powerful artificial illuminants. Our choice 

 is therefore limited to lime-light, gas and paraffin. 

 Of these, the latter is everywhere obtainable, gives 

 a white light, and is all that could be desired for 

 ordinary work with objectives up to -|- inch. Our 

 own arrangement consists of a paraffin lamp with 1^ 

 inch wick placed close to one end of an oblong zinc 

 reservoir. It is supported on a block about 20 

 inches from the stage. The glass chimney is narrow 

 to allow of the close approximation of a plano-convex 

 condensing lens of 3 inches aperture and 3 inches 

 focus, which collects the light and transmits it in a 

 slightly convergent beam. At a distance of 5 inches 

 from the stage is placed a second plano-convex 

 lens of 3 inches aperture and 5 inches focus. This 

 further converges the beam on the object, and gives 

 a brightly and uniformly illuminated disc of about 

 f of an inch in diameter, so that large objects can 

 be well photographed under low powers. The 

 object of using a short focus lens near the lamp is, 



