326 NOTES AND MEMORANDA. 



soaked with alcohol, and covered with a layer of collodion. When 

 tho collodion has acquired a sufficient consistency, very thin sections 

 may be made, comprising both the embryo and the plate of pith ; and 

 these are to be preserved in glycerine. If the sections cannot bo cut 

 directly, the piece is placed in the 40 per cent, alcohol ; the collodion 

 then preserves its consistency, and allows the embryo to be cut at any 

 time. 



This process is applicable to every kind of embryo of little thick- 

 ness, allowing it to be coloured en masse. It has the immense advantage 

 of enabling one to see at what level of the embryo each section is 

 made, to preserve it in the middle of a transparent mass, which main- 

 tains all the parts, and prevents their being damaged, as very often 

 happens when an inclusory mass is employed, from which the section 

 must be freed before mounting it. 



In his ' Precis de Technique Microscopique ' M. Mathias Duval 

 has already recommended collodion for embryological researches, but 

 without indicating his mode of employing it. We hope to render a 

 service to embryologists by making known a process which may be of 

 some utility. 



Improvement in Aerating Apparatus of Sea-water Aquaria.— 



Dr. H. Lenz, of Liibeck, has employed with success the following 

 method (suggested to him by Mr. A. Sasse, of Berlin) for producing 

 very minute air-bubbles from the aerating apparatus. The aperture 

 of the glass tube, instead of being drawn out into a fine point, is 

 widened to 6-8 mm., or a glass tube 25 mm. long and 6-8 mm. wide 

 is cemented with sealing-wax on to the short discharging arm. A piece 

 of common sponge is then pressed pretty tightly into the wide opening. 

 Instead of the somewhat large single air-bubbles, we then have 

 hundreds of very small ones in clusters, and the tighter the sponge is 

 pressed in, the smaller they become. 



By this means the air is as finely divided as by the syringe appa- 

 ratus of the large aquaria. Very slight, if any, increase of pressure 

 is found necessary ; and should in time algfe, &c., become attached to 

 the sjionge, it can easily be taken out and cleansed. Dr. Lenz used 

 his sponge for three months before it wanted cleaning.* 



Further Improvements in studying the Optical Characters of 

 Minerals. — Mr. H. C. Sorby has lately improved his method of 

 studying the optical characters of minerals. He says : f — 



" It is a curious example of how a method may be invented and 

 then lost sight of, that the determination of the index of refraction 

 in tho way I have previously described, was proposed by a French 

 savant upwards of a hundred years ago. I have not yet consulted the 

 original publication, but I very strongly suspect that the proposal was 

 more theoretical than practical, and that with the instruments then at 

 disposal the results were found to be so iuexact that the whole system 

 became obsolete and practically forgotten. I may, however, claim to 

 have so modified the method, and brought the instrumental means to 



* 'Zool. AnzuigLT.' ii. (1879) 20. 



t ' Mincralogical Magazine,' ii. (1878) 103. 



