292 



OPTICAL PROJECTION 



(the exact angles of course depend upon the glasses employed). 

 When the flint glass prism is used with the thick crown 

 glass prism, as at A in fig. 160, the apparatus gives refraction 

 with achromatism ; when the thin crown is used with the flint, 

 as at B in fig. 160, there is dispersion with an undeflected 

 beam. Black screens s can be adjusted to stop off any stray 

 parts of the beam ; or if it be desired, by adjusting one prism 



FIG. 159 



Fm. 160 



half of its height higher than the other, the effect of each 

 prism separately, and of the combination, and of the unde- 

 flected beam, can be shown together, this arrangement being 

 shown at c, fig. 160. 



More simply, a pair of small prisms to show achromatism, 

 as made for the Science and Art Department, and sold for 5s., 

 may be simply bound round with a rubber ring and stood 



