BISULPHIDE OF CARBON PRISM. 485 



the bottle into the funnel, to prevent its running over the rim of 

 the smaller funnel. The prism must not be filled completely, but a 

 space of a few cubic centimetres should remain empty ; if quite full, 

 it is difficult to close the aperture. This is done by placing a layer 

 of the above cement, very thin and about 2 mm wide, ronnd the aper- 

 ture, and upon this, after the cement has cooled but is not yet stiff, 

 a small piece of glass, about l cm square, is firmly pressed. 

 Whether the prism is tight may be judged by the absence of smell 

 if all carbon disulphide is removed from its vicinity. If no smell is 

 perceived, the piece of glass is covered several times with layers of 

 the cement and allowed to dry during several hours before it is 

 handled. 



If the prism during the filling in is seen to leak, the liquid must 

 be poured back into the bottle, the prism allowed to dry thoroughly, 

 and any leaky places must be covered again with the cement of 

 glue and treacle. 



The whole work is better performed in the open air ; the smell is^ 

 however, very soon got rid of if the windows are opened after the 

 prism is filled. 



If the work has been successful, the prism will have scarcely a 

 trace of smell, and a diminution of the liquid by evaporation will 

 not be perceptible for years. If it is to be cleaned and refilled, the 

 small glass plate may be removed with a knife, the liquid should 

 then be poured out and the prism placed in water to soften the 

 glue, so that the plates may be removed without too much force. 



The prism should be kept in the dark when not in use, for pure 

 disulphide of carbon, which is colourless, assumes a yellowish 

 tinge by the action of daylight, especially direct sunlight. The 

 yellowish liquid commonly sold is not suitable for our purpose ; the 

 student must procure the chemically pure substance. 



If a white object on a dark ground is looked at through the 

 prism, or a dark one on a bright ground, as for example a window 

 sash, it appears so much displaced that at first there is some diffi- 

 culty in finding it ; when found it presents itseF surrounded by very 

 broad beautifully coloured bands. 



White light consists of a mixture of variously 

 coloured rays, which combined produce the sensation of 

 white light. If the differently coloured rays which 

 impinge upon the prism were all equally deflected from 

 their original path, they would reach the wall after 



