36 Prof. Wood and Mr. Magnusson on 



structures of the nature defined, and available for crystals, is 

 therefore made up as follows : — 



No. of 

 Types- 

 Anorthic symmetry, destitute of coincidence-rotations 



or screw-spiral movement, furnishes 1 



Identical repetition of parts without mirror-image 



similarity furnishes 10' 



Mirror-image similarity superadded. 



With centres of inversion furnishes . 11 

 Without centres of inversion furnishes 10 



;} 



21 



Total ..... 32 



II. The Anomalous Dispersion of Cyanin. 

 By Prof. R. W. Wood and Mr. C. E*. Magnusson*. 



[Plates III. & IV.] 



ATTENTION has been already drawn by one of the 

 present writers f, to the very perfect prisms for the 

 exhibition of anomalous dispersion, that can be prepared by 

 pressing fused cyanin between plates of glass. It was shown 

 to be possible to prepare in this way prisms of any angle 

 desired, with perfect optical surfaces, a thing which it is 

 quite impossible to do by the evaporation method employed 

 by Wernicke and Pfluger. In the same article was given a 

 table of refractive indices for different wave-lengths, and the 

 dispersion-curve in so far as it was studied. At the time of 

 the publication of this paper no satisfactory readings within 

 the absorption-band had been made, owing to the difficulty 

 of pressing prisms of small enough angle to transmit the 

 strongly absorbed rays. Readings within the absorption- 

 band have been made by Pfluger with prisms obtained by the 

 evaporation of an alcoholic solution of cyanin, and the dis- 

 persion-curve found to be continuous, but the surfaces of his 

 prisms were necessarily so imperfect, that it seemed important 

 to repeat the work under better conditions. 



Since the publication of the paper alluded to above, the 

 work has been continued by the present writers, with still 

 more perfect prisms, very satisfactory readings having been 

 obtained not only in the region of the absorption-band, but 

 also in the ultra-violet. 



* Communicated by the Phj^sical Society, 

 t Wood, Phil. Mag. xlvi. pp. 380-386. 



