134 TRANSACTIONS OF THE 



should not be used from the mirror, but should be reflected from 

 a piece of plate glass, under which fine black silk velvet has been 

 laid. As the ordinary white light is often dispersed into prismatic 

 colours by the nature and form of the objects themselves, resulting in 

 impaired definition and resolution, this difiiculty may be overcome 

 by the use oi monochromatic light or light of one colour. Such light 

 may be obtained from the spectrum, or by passing the white light 

 through coloured glass or through a flat glass cell containing an 

 ammoniacal solution of sulphate of copper, the blue and violet 

 rays being the most suitable owing to the greater length of their 

 waves. Such coloured glasses or cells containing coloured solutions 

 are called light modifiers. As daylight, and especially sunlight, 

 are scarce commodities in this country, at least in our large cities, 

 and as the amateur microscopist has generally to pursue his 

 observations in the evening after his day's work is done, recourse 

 must be had to artificial light, which indeed, especially for high 

 powers, is the handiest, and suffices for all but the resolution of a 

 few of the most difficult tests. Gaslight is not very suitable 

 owing to the ever varying pressure in the main, in consequence of 

 which the flame does not burn steadily enough. An ordinary 

 paraffin or camphine lamp for one shilling will do very well as far 

 as light is concerned, but I recommend to you Hovos Microscope 

 Lamp and also the so-called Bochet Lamp. How's lamp consists 

 of a glass reservoir holding sufficient oil to last for foui- to flve 

 hours, and is adjustable on a brass rod fixed in a large circular 

 foot. A ring of stout wire holds one of Hale's porcelain shades, 

 which has only one opening in front to allow of the passage of 

 the rays ; this shade keeps in the heat, protects the eyes from the 

 glare, and reflects a white light. Every one using a microscope 

 regularly, even if it be only a small student's microscope, should 

 provide himself with one of these lamps ; he will find that the few 

 shillings it costs are well invested. A handsomer lamp is Swift's, 

 which packs in little space and has an additional reflector and 

 bull's-eye condenser with blue light modifier turning on the same 

 arm. The glass stopper in the reservoir for introducing a further 

 supply of oil when necessary is, in my opinion, no great improve- 

 ment, as the oil oozes out when the lamp gets hot. Fiddian's 

 Microscope Ijamp has a metallic chimney which is internally coated 

 with plaster of Paris, and has a white cloud reflector, neutral 

 tint-shade, &lg. It is the most complete and best lamp made, but 



