142 TEAR-BOOK OF FACTS. 



Miss King, now the Honourable Mrs. Ward. When the decompo- 

 sition has gone regularly on round a single point, and there is no 

 other change, there is a division of the glass into a number of 

 hemispherical films within one another. The groups of films ex- 

 hibit in the microscope circular cavities, which, under different cir- 

 cumstances, become elliptical and polygonal. M. Brame, of Paris, 

 succeeded in rapidly producing this composition by immersing glass 

 in a mixture of fluoride of calcium and concentrated sulphuric acid, 

 or by exposing it to the vapour of fluohydric acid ifiomptes Eeudus, 

 Nov. 2, 1S52). 



The author then proceeded, with the diagrams, to explain the 

 optical phenomena, grouping them into three chief varieties, but 

 stating them to be so various and singular as to baifle description. 



Many other optical circumstances connected with this .variety 

 •were mentioned by the author and explained. In all these three 

 varieties the films are pure glass, for they become colourless by a 

 sufficient inclination of the plates, and also by introducing a drop of 

 ■water or alcohol, which, when it evaporates, allows the original 

 colours again to be recovered ; and although a film of the fluid sepa- 

 rated eacli of the almost infinitesimal layers of the glass, yet they 

 afterwards adhere as firmly as ever. If an oil or balsam be intro- 

 duced, it slowly and unequally passes between the layers, so that 

 the retreating colour is bounded by a stratum of the various tints 

 * which the film combines. But the author has often found between 

 the true glass films beautiful circular crystals of silex, finely seen 

 in polarized light. These are sometimes dendritic, and assume, 

 round the black cross, foliated shapes. One form merits particular 

 attention : around a minute speck of silex there is formed a circular 

 band of equally minute crystalline specks, and at a greater distance 

 a second circular band concentric with the first, consisting of still 

 smaller siliceous particles hardly visible in the microscope. By 

 what atomic forces does this central crystal group its attendant 

 crystals around it ? 



Mr. Stoney observed that Dr. Lloyd, at the Aberdeen meeting, 

 had shown that the light from these specimens of decomposed 

 exhibited by Sir D. Brewster, was elliptically polarized, and that 

 therefore they must behave like uniaxal crystals. 



STATISTICS OF COLOUR- BLINDNESS. 



Sir .loiiM Hkkschkl observes : — "Dr. Wilson* gives it as the 

 result of his inquiries, thai tin every eighteen is colour- 



blind in some masked degree, and that one in fifty-five confounds 

 red with green. Were the averagi like this, it seems in- 



rable that the existence of the phenomenon of colour-blindness, 

 or dichromy, should not be one of vulgar notoriety, or that it Bhould 

 strike almost all uneducated persons, when told of it, as something 

 approaching to absurdity, nor can I think that in military i 



(as, for instance, in the placing of men as sentinels at outposts),. 

 • See Year-Book of Facts, pp. 150, 161. 



