332 NOTES AND MEMORANDA. 



of liquid is tlicn removed by blotting-paper, and thereby a slight 

 and advantageous pressure produced on the cover-glass. 



After the cover-glass is in place, two of the opposite sides should 

 bo fastened either with paraffin or Canada balsam to prevent displace- 

 ment in colouring. 



To colour the organisms he uses eosin or Eanvier's picro-carminate. 

 Infusoria previously treated with the osmic acid may be coloured 

 direct with the picro-carminate, but when it is employed alone, it is 

 not easy to control the colouring, so that the preparations often turn 

 out opaque. After several attempts, he found that a mixture of glyce- 

 rine and picro-carminate will enable any degree of colour to be obtained 

 (glycerine 1 part, water 1 part, picro-carminate 1 part). Introduced 

 suddenly, the glycerine even when diluted frequently produces an 

 abnormal retraction of the tissues, which does not always disappear. 

 Professor Ranvier gives in his ' Histology ' a very simple means of 

 avoiding this inconvenience, which M. Certes has employed with 

 success for the most delicate organisms, such as OxijtricTia and Stentor ; 

 it consists in placing the preparations, fastened as above described, in 

 a moist-chamber, and putting a drop of carmiuated glycerine on the 

 edge of the preparation. The water evaporates very slowly, and 

 in twenty-four hours is replaced by the diluted glycerine. By the 

 same process the latter may be replaced by concentrated glycerine, 

 which assures the preservation of the preparations. 



All methods of sealing down may be applied. It is, however, 

 better to use dry Canada balsam dissolved in chloroform. The 

 organism to be examined might be at the side of the glass, and this 

 varnish, being thin and perfectly transparent, does not hinder observa- 

 tion even with the highest powers. 



Mixture of Oils for Homogeneous-Immersion Objectives. — Pro- 

 fessor Abbe points out that in regard to the performance of oil- 

 immersion lenses with central light it is a matter of importance to 

 regulate carefully the oil-mixture as regards refraction and dispersion. 

 He noticed some time ago that some of the samples of fennel-oil and 

 olive-oil were rather strong in both respects, so that it is possible that 

 better performance will be got with central illumination when a 

 small additional quantity of olive-oil is added for reducing the 

 refraction to that of the oil of cedar-wood, and then further adding 

 if or I of cedar-oil to the mixture to reduce the dispersion (the latter 

 specially for thin covers). 



New Fluids for Homogeneous Immersion. — The result of 

 Professor Abbe's later experiments will be found at p. 346 of the 

 ' Proceedings.' 



Standard Micrometers. — A letter from Professor R. Hitchcock 

 (the editor of the ' American Quarterly Microscopical Journal ') on 

 this subject is printed at p. 349 of the ' Proceedings.' 



Unit of Micrometry. — The resolution come to by the meeting 

 of the Society on the 9th April will be found at p. 349 of the 

 ' Proceedings,' 



