THE BOOK OF THE LANTERX. 183 



bisulphide of carbon prism. This takes the form of a 

 stoppered bottle with two sides ground away and filled in 

 with plates of glass, which are cemented to the re- 

 mainder of the bottle. In this way the wedge form 

 of the prism is secured. The bottle is then filled with 

 bi-sulphide of carbon, and such bottles, ready charged, can 

 be obtained at the opticians'. A great objection to them 

 is their liability to breakage, for bi-sulphide of carbon, 

 beyond being a most inflammable compound, has a most 

 disagreeable and pungent odour. 



In using a prism of this description, it is kept upright 

 and supported in front of the lantern. The slit in the card 

 must in this case be vertical, instead of horizontal, and the 

 lantern must be placed at such an angle with the sheet 

 that when the spectrum is rendered visible it appears in a 

 central place on the sheet. 



There are several means available for showing on the 

 lecture-table that the various colours of the spectrum will, 

 when combined, once more form white light. Thus we 

 may place in the path of the coloured beam a double con- 

 vex lens, which will at once bring the scattered rays to a 

 focus, and will form a disc of white light. We can also 

 recompose light by collecting the coloured rays by means 

 of a concave mirror, when a card held in the focus of the 

 mirror will exhibit a brilliant spot of light free from 

 colour. Another method is to use two prisms placed 

 against one another, thus Ay> when one will neutralise 

 the effect of the other, and the emergent beam will be 

 white. Yet another way of recomposing light is to use a 

 number (generally seven) of plain mirrors, which are so 



