New Method of exhibiting Anomalous Dispersion. 625 



gets through the prism, and on viewing a source of light 

 through the refracting edge we see merely a red and a 

 blue image, the former beino- deviated more than the latter. 

 With a new supply of the dye which we have just received 

 I have, however, been able to make prisms of over one degree, 

 which transmit an abundance of green light. Viewing the 

 incandescent loop of an electric lamp through one of these 

 prisms, we see a most beautiful anomalous spectrum — a broad 

 band of light with the colours arranged in the order green, 

 blue, violet, red, and orange. When it is remembered that 

 the largest angle which Pfliiger was able to obtain by 

 Wernicke's method was of but two minutes, the advantage 

 of the fusion-method is apparent. 



While engaged in some experiments on the dispersion of 

 selenium, from which most beautiful prisms can be made by 

 the same method, I was led to try the experiment of crossing 

 one of these prisms with a small diffraction-grating. Selenium 

 has an extraordinarily high refractive index, over 3 for certain 

 colours, and the prisms are quite transparent for the red and 

 orange. The deviation produced by one of these prisms is 

 about double the angle of the prism, and I was led to try the 

 experiment of crossing one of them with a diffraction-grating. 

 On viewing an arc-lamp through the combination, the dif- 

 fraction-spectra were most distinctly seen on each side of the 

 central image, each one with its tail nicely curled up at the 

 edge of the absorption-band, which begins in the yellow and 

 stretches to the extreme ultra-violet. 



Having such excellent cyanine prisms at my disposal, 

 it occurred to me to try crossing one of these with a 

 diffraction-grating, for the purpose of showing the dispersion- 

 curve. I have usually used a spectrometer and low-dispersion 

 prism for this purpose. The grating mounted with the cyanine 

 pri c m was found to be equally efficient. One has only to view 

 an arc-light through the combination. The diffraction-spectra 

 are deviated by the prism, the red ends being turned up, while 

 the blue-green ends are turned down in a most beautiful 

 manner, 1 used a photographic copy of a 2000-line-to-the- 

 inch grating, about 5 mm. square, fastened over the refracting 

 edge with sealing-wax. 



The curved spectra can also be seen when the sun is viewed 

 through the combination, though less perfectly, owing to its 

 size. 



In conclusion, some hints regarding the construction of 

 cyanine prisms may be of use to any wishing to repeat the 

 experiment. 



The cyanine was obtained from Giubler of Leipzig, and is 



