248 Mr. A. E. Tiitton. Crydallographical Study of the 



prismatic cameras are not included, for the reason that the reduction 

 of the photographs is not yet completed. 



The second part, by Captain Chisholm-Batten, E.N., gives a full 

 account of the observations made by the officers and men of H.M.S. 

 "Melpomene"; four photographs of the corona, taken with different 

 exposures, accompany the paper. 



In the third part, Professor Pedler gives details of the spectroscopic 

 observations which he made by eye with a 6-inch short focus lens and 

 a grating spectroscope. 



A general idea of the work at Yiziadrug has already been given in a 

 preliminary report.^ 



" A Comparative Crystallographical Study of the Double Selenates 

 of the Series EoMCSeOJ.^eH^O.— Part I. Salts in which M 

 is Zinc." By A. E. Tuttox, B.Sc, F.R.S. Pteceived March 5, 

 —Bead March 15, 1900. 



(Abstract.) 



In this communication are presented the results of the investigation 

 of the group of salts of the above series in which M is represented by 

 zinc, R being represented by potassium, rubidium, and caesium. The 

 investigation is similar to that which has previously been carried out 

 for the double sulphates of the analogous series.! The work consists 

 of very large numbers of measurements of the exterior angles of the 

 crystals, determinations of density, refractive index, optic axial angle, 

 orientation of optical ellipsoid, and effect of change of temperatiu?e on 

 the optical properties, together with the calculation of all the morpho- 

 logical and physical constants derivable from the measurements. The 

 main results are as follows : — 



The morphological axial angle of the rubidium zinc salt is approxi- 

 mately the mean of the axial angles of potassium zinc and caesium zinc 

 selenate. 



In the cases of thirty-three out of thirty-six angles between the 

 exterior faces, the value for the rubidium zinc salt is intermediate 

 between the values for the other two salts, and the exceptions are only 

 apparent, being due to changes of opposite sign in adjacent angles 

 following the rule. 



The morphological axial ratios for rubidium zinc selenate are inter- 

 mediate between the ratios of potassium zinc and caesium zinc selenate. 



The common habit of the crystals of the rubidium zinc salt is of an 



* ' Roy. Son. Pj oc' vol. 64, p. 27. 



t * Journ. Chem. See, Trans.,' 1893, 337, and 1896, 344. 



