PROCEEDINGS OF THE SOCIETY. 491 



should be plunged in a, liquid of the same index as the cover-glass, 

 glycerine mixed with oil of aniseed, for instance, or even the latter 

 alone ; later he recognized that distilled water corrected very well 

 the feeble aberrations produced by the differences in the relative thick- 

 ness of the media, glass and water. . . . The experiments of Amici 

 on immersion objectives date from 1844. I saw in that year, or the 

 following one, at Oberhlluser's, an objective with which he showed 

 the advantages [of the immersion system] by interposing between it 

 and a preparation of Lepisma scales either a drop of neats' foot oil or 

 a drop of essential oil. He considered them to be preferable to 

 water, the employment of which he had already recommended as 

 giving good results with all kinds of objectives of short focus." * 



Mr. Woodall called attention to the passage in Professor Abbe's 

 paper (see p. 256), in which he referred to Mr. Stephenson's sug- 

 gestion, that homogeneous immersion objectives would allow of 

 increased angular aperture, which suggestion led to the making of the 

 objectives. 



, Mr. Ingpen thought they must all regret that Mr. Wenham had 

 not followed up his experiments in homogeneous immersion. Had he 

 done so, we probably should have long since had the oil-immersion 

 objective as an English instead of a foreign production. The refrac- 

 tive index of the immersion fluid employed was not the only con- 

 sideration — the selection of an oil of suitable dispersive power had 

 been made by Professor Abbe, after many experiments. Moreover, it 

 must be remembered that the great advantage of the oil lens was its 

 increased angle and consequent augmented resolving power, which 

 was not originally contemplated as a result of homogeneous immer- 

 sion. In the case of the new lenses they had not merely the results of a 

 series of experiments, but also their successful practical application in 

 the construction of improved objectives. 



Mr. "Wenham wished to say one word as to the medium. At the 

 time referred to he had used oil of cloves. He did not care to make 

 any oil lenses then because he had a wholesome fear of it. If the 

 fingers were smeared with it and the instrument then touched, it took 

 off the lacquer, besides unsetting the cement and destroying the 

 objects. 



Mr. Hue's suggestion for the more convenient use of oil with 

 homogeneous immersion objectives was explained by Mr. Crisp, 

 viz. to screw over the front of the objective a small receptacle contain- 



* Prof. Ch. Eobin, ' Traite du Micmscope.' Paris, 1871. Pp. 191-192. 

 Prof. Harting, in liis work on the Microscope, thus referred to the use of oil : — 

 " If we could replace the layer of water by a fluid of still greater refractive 

 power, such as oil, further advantages must obviously be obtained. This ha.s 

 been successfully tried. It seems to me, however, a great risk to bring costly 

 objectives in contact with an oily fluid which would have to be again removed by 

 alcohol and ether. This would be hazardous with double lenses cemented with 

 Canada balsam. The immersion system has, it is true, been so arranged that the 

 front lens is not a double, but a single one of crown glass, and for these immersion 

 in oil would certainly be much less objectionable. Nevertheles.s, I must doubt 

 whether the oil-immersion system can ever come into more general use." — P. 

 Harting, ' Das Mikroskop' (2nd German edition). Brunswick, 1866. 



