Anomalous I )/sj tersion of Cyan/// . ; > <s 3 



aperture in such a way that the refracting edge is in contact 

 with one side of the aperture. This screens off all rays 

 except those which pass through the cyanin. If the telescope 

 of a spectrometer is directed towards the slit of the instrument 

 on which is focussed an image of the sun or the electric arc 

 and the prism put in place, a most beautiful anomalous 

 spectrum will be seen, bluish green being the least deviated, 

 followed by blue and violet, a wide dark space, orange and 

 red. I have even succeeded in projecting this spectrum so 

 that it could be seen by a large audience. To do this requires 

 sunlight, a broad beam of which should be focussed on a narrow 

 slit, and a projecting lens so placed as to throw an image of 

 this slit on a screen 3 or 4 metres distant. The image of 

 the slit will of course be 3 or 4 centim. wide on the screen. 

 On placing the cyanin prism in front of the lens the ano- 

 malous spectrum will appear, considerably blurred and rather 

 dim, but clearly recognizable in a sufficiently dark room. By 

 carefully selecting the prism I have succeeded in producing 

 on the screen a spectrum measuring about 15 centims. in 

 length, showing colours in the order green, blue, and red 

 with perfect distinctness. 



The oppositely curved portions of the spectrum seen by the 

 method of crossed prisms, and described by Kundt, can be 

 showm very nicely with a prism of solid cyanin mounted on a 

 black card as already described. The slit of a spectrometer 

 should be covered with tinfoil with the exception of about a 

 millim., and sun or electric light focussed on the clear space. 

 A low-dispersion prism (best a hollow prism filled with 

 water) should be set on the table of the instrument and the 

 narrow continuous spectrum brought into the field of the 

 telescope. On holding a 15 7 cyanin prism in front of the 

 objective of the telescope with its refracting edge horizontal, 

 the yellow portion of the spectrum will be removed, and the 

 remaining portions curved in opposite directions, the curves 

 being identical with the dispersion-curve plotted from direct 

 observation, except that in the latter case the spectrum is 

 normal. 



The great advantage of prisms prepared in this way over 

 those made by evaporation is their large angle and excellent 

 optical surface. Pfliiger worked with prisms varying from 

 one to one and a half minutes, two minutes being about the 

 maximum angle he could produce satisfactorily. It appearing 

 probable that a much more accurate determination of the dis- 

 persion could be obtained by means of prisms of much larger 

 angle, I arranged a piece of apparatus as follows : — The light 

 of the sun from a heliostat was focussed on the slit of a very 



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