LIGHT: REFRACTION 



281 



under pressure. A two-necked glass receiver, as shown in 

 fig. 148, will answer very well, blacked all over except for a 

 circle of four inches diameter opposite one orifice. This is 

 supported as at A. The orifice is a short piece of glass tube 

 fixed in cork, upon which the light will be condensed. Several 

 feet higher the supply of water must be placed (a bucket will 

 suffice) connected with the receiver by the tube B, and the 

 receiver is filled. On withdrawing a cork from the glass tube 

 at the orifice, the water will issue in a smooth stream, when 

 the radiant can be adjusted to give the best effect. Various 

 colours can be imparted to the stream by placing different 

 coloured glasses in the slide stage of the lantern. 



Instead of this primitive apparatus, a vessel of tin, with 

 one glass side to go against the lantern flange, and an orifice 

 opposite, may be constructed. Such a vessel will be supported 

 more steadily, and the light can be more prc cisely adjusted 

 through a plane glass ; while the pencil attachment shown 

 in fig. 95 may be used to give more brilliance. 



159. Prisms and Lenses. The permanent deviation of a 

 ray or beam of light 

 by a prism is best 

 shown by placing a 

 small aperture in 

 black card or metal 

 in the stage and 

 focussing on the 

 screen (or a hole in 

 the plate of aper- 

 tures focussed by 

 the loose lens will 

 do equally well), and 

 interposing in the 

 path of the rays a thin prism, often called a wedge prism, 

 which will show no conspicuous colour. Such a prism may 

 be made by cementing a wedge-shaped glass cell, and filling 



FIG. 1 4 J. Spectrum 



