The path of progress in microscopy indicated by the homogeneous-immersion 

 system was clearly in the direction of higher and still higher apertures, till 

 the limit should be reached — the limit imposed by the refractive index of 

 the front lens, the immersion medium, or the cover glass. It was gratify- 

 ing to reflect that our leading microscope opticians, Jlessrs. Powell and 

 Lealand, had fully realised the importance of this progress, and had steadily 

 extended the apertures of their high powers until about three years ago 

 they arrived very approximately at the limit imposed by the plane surface 

 of the cover glass. When that point was reached, which might fairly be 

 said to obtain when the apertures of microscopic objectives were extended 

 to within 2J per cent, of the transmitting power of the plane surface of the 

 cover glass, it seemed that the only further progress to be made would con- 

 sist of slight changes in the formulae -whence minute reductions in the 

 residuals of aberration might be effected, and these improvements, com- 

 bined with higher quality of technical execution of the lenses, centering, 

 &c., would lead to somewhat greater perfection of images seen in the 

 microscope. But, happily, the ultimate progress of microscopy, so far as 

 the perfection of the microscope itself was concerned, was not really so 

 restricted by the nature of things ; and this conception got possession of 

 Dr. Abbe's thoughts to such purpose that he was induced to make large 

 sacrifices of time and means to fairly test the matter, for he had reasoned 

 himself into the conviction that by an exhaustive series of experiments it 

 might be possible to discover new compounds of glass' by which a higher 

 degree of achromatism might be obtained, thus furnishing a new basis or 

 starting-point for improvements in all optical instruments. With the 

 assistance of Dr. Otto Schott, an able chemist who had had much ex- 

 perience in the manufacture of glass. Dr. Abbe carried out a long and 

 carefully-classified series of experiments, resulting in the discovery of new 

 kinds of glass, by which higher achromatisation than any previously 

 reached was plainly demonstrated in practical constructions. These new 

 kinds of glass formed the basis of the new microscope-objectives termed 

 " Apochromatic," and for the " compensating " eyepieces recently issued 

 by Messrs. Zeiss, the well-known opticians of Jena, and since taken up 

 with important modifications by Messrs. Powell and Lealand, of London, 

 Mr. Mayall explained in detail the aim of the new apochromatic objectives 

 and the points of difference when compared with the older objectives, in 

 the course of which he described the circumstances that led him to \'isit 

 Dr. Abbe at Jena, with whom he had examined a large number of microscope 

 objectives that might be regarded as representative of the highest points of 

 technical excellence reached in England and Germany. He mentioned with 

 special pleasure the fact that when he informed Dr. Dallinger, the President 



