Xew Optical Property of Biaxial Crystals. 513 



from the crystal depends on the refractive index and hence 

 must make a greater angle with the normal to the plate for 

 the shorter wave-lengths. This is exactly what is actually 

 observed. In an exact quantitative discussion the fact that 

 the direction of the axis of single-ray velocity in the crystal 

 varies with the wave-length must also be taken into account. 

 A. fuller discussion of this point and of the resolving power 

 of the crystalline plate regarded as a spectroscope in itself 

 would be of interest. This would obviously require a deter- 

 mination of the distribution of luminosity in and around the 

 point of singularity in the wave-front. It is hoped at an 

 early opportunity to carry out a detailed investigation on 

 these points. 



3. Summary and Conclusion. 



(a) The paper describes a new optical effect observed 

 with a biaxial crystalline plate, viz., that such a plate 

 though bounded by parallel faces, is capable of forming 

 on one side of it when suitably oriented a real erect image 

 of a source of light such as a luminous filament placed at 

 some distance from it on the other side. 



(b) The characteristics of this image-forming property 

 are set out in detail, a noteworthy feature being the spectral 

 resolution of the image which occurs when either the object 

 or the image is at a considerable distance from the crystal. 



(c) A theorerical explanation ol the phenomenon is sug- 

 gested, viz., that the singular point in the Fresnel wave- 

 surface is* a locus of maximum intensity, and, owing to the 

 fact that the wave-form on emergence from the crystal 

 retains its general character, we get a continuous concentra- 

 tion of luminosity along a line any point of which is in effect 

 an optical image of the source. 



The investigation described in this paper was carried out 

 in part in the senior author's laboratory at Calcutta, and 

 in part at University College, London, during the visit to 

 England of the senior author, who wishes to express his cordial 

 thanks to Prof. A. W. Porter, F.P.S., for his hospitality 

 and kind interest in providing the necessary facilities for 

 the work. 



Phil. Mag. S. 6. Vol. 43. No. 255. March 1922. 2 L 



