482 



BISULPHIDE OF CARBON PRISM. 



any white light appears : this coloured band is called a 

 spectrum. The colour which is least refracted and appears 

 therefore lowest in our spectrum is the red; the upper- 

 most colour, which is most refracted, is the violet. 



FIG. 277 (A, an.proj.; A and B, f real size). 



The prism should be made of the wider part of a lamp cylinder ; 

 a broken cylinder may be used, as only a short piece is required. 

 First draw with ink the two lines which form the edges of the prism 

 at each end, and along which the cylinder is to be cut, taking care 

 that the two ellipses thus drawn appear each as one line, if seen by 

 an eye kept in the same plane, and also that their inclination be that 

 in the figure shown at 5, viz. 45, so that the refracting angle may 

 have the desired magnitude ; for if the angle is too small, the spectrum 

 is too short, and if the angle is too great no spectrum is produced, 

 because in that case the light does not emerge from the second re- 

 fracting surface, but is reflected backwards. When the lines are 

 quite dry, a crack from the edge is made with pastille and carried 

 round along the line ; the same is done at the opposite end. The 

 edges must be carefully ground with emery-powder upon an iroii 

 plate ; but, before this is done, the hole for filling the prism with 

 liquid should be bored with a file, for otherwise, if the glass should 

 break while the hole is being made, the labour of the grinding would 

 have been thrown away. The plates which form the refracting 

 edges must be made of perfectly flat plate glass, or the prism is 

 nearly useless. If such glass cannot easily be obtained, the silvering 





