PROCEEDINGS OF THE SOCIETY. 007 



said, especially in his advocacy of all that tended towards the perfec- 

 tion of objectives. As regarded the application of the correction- 

 adjustment to homogeneous-immersion objectives, he was obliged to 

 express his disagreement with Dr. Dippel. The facts on which his 

 own judgment on this question was based were briefly these : — When 

 Zeiss's homogeneous-immersion objectives were first sent to England, 

 he immediately observed that certain dry objects, such as Podura, 

 were very indifferently defined by the new lenses, precisely as he had 

 previously found with water-immersions in fixed settings. These 

 objects were always such as did not adhere very closely to the cover- 

 glass. Knowing from, experience that the correction-adjustment had, 

 in many cases, met the difficulty with water-immersions, he took an 

 early opportunity of pressing upon Messrs. Powell and Lealand to 

 apply the adjustment to the homogeneous-immersions. The result 

 fully answered his expectations. There could be no doubt whatever 

 that the correction-adjustment increased the range of conditions 

 within which the homogeneous-immersions would give fine definition. 

 He might state, as a matter of repeated personal experience, that in 

 testing a number of homogeneous-immersions — fixed settings versus 

 adjustment settings — whilst there were undoubtedly many prepara- 

 tions which were defined equally well by both systems, there were 

 also many upon which no superficial structure could be discerned 

 with the objectives in fixed settings, but which yielded well-defined 

 images when viewed with objectives having the correction-adjustment. 

 The differences in the lenses might not be so marked as those seen in 

 comparing dry lenses with and without adjustment ; but still they 

 were unmistakable, and unquestionably in favour of the adjustment. 

 In the best homogeneous-immersion objectives he had examined, the 

 corrections were so sensitive that different thicknesses of cover-glass 

 had to be compensated for by the adjustment ; whilst different speci- 

 mens of oil of cedar-wood so completely altered the character of the 

 image, that unless the correction-adjustment were brought into use, 

 the objective might be condemned as defective. Even with the cor- 

 rection-adjustment, a marked variation from the normal immersion- 

 fluid could not be compensated for. The objectives he here referred 

 to were those of apertures from 1*2 to 1*47. He could not agree 

 with Mr. Ingpen that it was best for the amateur to let the opticians 

 choose for him the best average adjustment, and there fix the lens- 

 mounting. He considered the amateur should make himself skilled 

 in the use of the adjustment. As to the difficulty of finding two 

 persons who would agree on the best point of adjustment in attempting 

 to interpret an image of an histological preparation, he thought the 

 solution of the difficulty would be best found by ensuring greater skill 

 in making the preparation. When the Bacillus tuberculosis was first 

 observed, it was only after great perseverance that anything could be 

 interpreted from the confused mass of images ; but the moment better 

 methods of treating the preparations were found, the difiiculties 

 vanished, and what had formerly required hours of patient investiga- 

 tion to glimpse was now exposed to the eye at a glance. It would be 

 interesting to the Society to learn that Prof. Abbe himself had so far 

 wavered from his former opinion against the application of the cor- 



