38 NOTES AND MEMOKA.NDA. 



the chromatic and spherical aberrations are corrected by the central 

 lens of dense flint. This lens is nearly three times as thick as the 

 crown-glass lenses. The interior curves are almost hemispheres. 

 The final correction for spherical aberration is made by altering the 

 thickness of the dense flint-glass lens. The three lenses are united 

 by a transparent cement which has a refractive index corresponding 

 very nearly with the glass. This prevents light being lost by re- 

 flexion from the surface of the deep curves. High-power Stanhope or 

 Codrington lenses are only suitable for the examination of transparent 

 objects. The Platyscopic Lens focusses about three times as far from 

 the object as the Stanhope or Codrington lenses. This allows opaque 

 objects to be examined easily, as light can be conveniently allowed to 

 fall upon them at any required angle. 



Lactate of Silver as a Colouring Agent. — It seems that M. 

 Alferow * advocates the use of the lactate of silver instead of the 

 nitrate. He recommends a solution of one part in eight hundred of 

 distilled water, with the addition of a few drops of a concentrated 

 solution of lactic acid. The presence of the free acid renders pre- 

 cipitation less easy, and the only formations that occur are the 

 chloride and albumicate of silver. The formation ot several disturb- 

 ing precipitates is thus avoided. Alferow asserts that the pictures 

 are much clearer than those obtained by the nitrate, and that the 

 lactate, if applied to the mesentery of the living frog, interferes less 

 with the circulation. It may be mentioned that this author denies 

 the existence of stomata in blood-vessels and serous membranes, 

 and states that solid particles traverse epithelial layers between the 

 cells, which move to give them passage by some mechanism not yet 

 understood. 



Silica Films and the Structure of Diatoms. — Mr. G. W. More- 

 house has been following out the line of investigations begun some 

 time ago by our Secretary, Mr. H. J. Slack. In a paper read before 

 the Memphis (U.S.A.) Microscopical Society, at one of its summer 

 meetings, the writer remarked that he had prepared some films of 

 silica, after the process described by Mr. H. J. Slack.j " To facilitate 

 examination, some of the films were carefully washed and then 

 mounted in balsam ; others were burned out upon the glass cover 

 before mounting. The latter method is much the best and quickest. 

 The opinion of so distinguished an observer as Mr. Slack is entitled 

 to great weight, and the writer is happy to be able to concur with him 

 in the opinion that the cellular character of some of the films is due 

 to bubbles of gas ; and that, under the conditions to which these 

 experiments are necessarily confined, the deposition of the silica is 

 generally in the form of spherules. No one pretends that these con- 

 ditions approximate to those that obtain in the growth of the protec- 

 tive covering of the living diatom ; yet, in a kind of unexplained 

 general way, the experiments upon these artificial films are supposed 

 by some to strengthen the ' bead theory,' although the conditions 



* ' Archives de Physiologie,' Nos. 4 and 5, 1874. 



t ' Monthly Microscopical Journal,' .Jnne 1, 1874, p. 238. 



