G. GENERAL NATURAL HISTORY AND ZOOLOGY. 273 



over the steam again until the crystals have grown a trifle 

 larger, and so on until the specimen is satisfoctory. The 

 specimens are to be first varnished over with a film of col- 

 lodion, and then mounted in old Canada balsam. American 

 Naturalist^ May^ 1876. 



ORIGIN OF THE RED CLAY OF DEEP-SEA SOUNDINGS. 



Dr. Carpenter, at the Bristol meeting of the British Asso- 

 ciation, questioned the theory of Dr. Wy ville Thompson on 

 the origin of the red clay universally found in the deepest sea 

 soundings. Dr. Thompson supposed it the residue, after the 

 calcareous portion of the shell had been dissolved by the ex- 

 cess of carbonic acid, the mineral matter not calcareous left 

 behind being a red silicate of iron ; and in support of this 

 view Dr. Thompson stated he had obtained a similar deposit 

 of " red ash," as he terms it, from the ordinary foraminiferous 

 ooze, after removing the calcareous portion by dilute acid. 

 Dr. Carpenter, referring to the discovery of casts of the fo- 

 raminifera in the greensands, and especially as noted by the 

 late Professor Bailey, considers that the red clay, instead of 

 being the ash from the foraminiferous shells, was but the 

 hio'her oxidization of the iron of the internal casts, and the 

 disintegration of them by the action of carbonic acid. The 

 casts are formed by the decomposition of the animal, when 

 the silicates precipitated from sea-water take the place of 

 the animal substances, particle by particle, filling completely 

 the cavities of these minute shells with green or ochrey sili- 

 cates. 



SOEBY ON LIMITS OF VISION. 



Mr. Sorby's address, at the annual meeting of the Royal 

 Microscopical Society, on the ultimate limits of the micro- 

 scope, as shown by the formula of Helmholtz, has elicited a 

 reply from Count Castracane. He states that the resolution 

 of the nineteenth band of Robert's test -plate exceeds the 

 limit determined by the formula, and Mr. Sorby is called 

 upon to explain the discrepancy. Mr. Sorby does not per- 

 ceive any serious difiiculty in explaining on Helmholtz's 

 principles the resolution of the band in question, and he 

 states that it is probable, w^ith such an illumination as that 

 adopted by Count Castracane, that the interference fringes 



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