ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 291 



under similar treatment by coarser objects, of whicb true dioptric 

 images can be obtained. But even then it will be only a matter of 

 interpretation more or less correct. All we know of A. pellucida is, 

 that there are strise, or ridges, or something occurring at intervals of 

 so many to the inch. It is obvious they cannot be mere lines, as they 

 appear to us ; but as any " form " must involve another set of lines 

 of at least double the minuteness, and probably far more, what is 

 there can never be known, except from analogy and comparison with 

 larger diatoms. As a rule, we must get lines only in minute 

 structures, and as a rule, that appearance is certainly false. Never- 

 theless, the variations in distance between the spectra under gradually 

 increased obliquity, and their consequent image-results, appears the 

 most likely general method of ascertaining the true proportions of 

 distances between striae not all equidistant, while the successive 

 stopping out of the inmost and brightest spectra appears the most 

 promising general method of revealing those fainter spectra which 

 may lie hidden behind, and which reveal some periodic variation in 

 structure at the distances of the apparent lines. 



The Future of the Microscope. — Amongst the reports on the 

 South Kensington Loan Collection of Scientific Apparatus is one by 

 Prof. Abbe on the " Optical Aids to Microscopy," * the earlier part of 

 which (pp. 383-91) is occupied with a general description of the 

 stands, objectives, and other apparatus exhibited, with critical com- 

 ments. The succeeding thirty pages are devoted to a consideration of 

 " the facts which throw a light on the conditions of optical perform- 

 ance, and furnish hints in regard to further progress."! 



The author commences with expressing the opinion that no epoch- 

 making advance in the way of an extension of the domain of micro- 

 scopical perception is now possible, although there is still great room 

 for improvement in other, and in a relative sense minor, respects. 

 The curve of progress, after having risen abruptly for several decades, 



* Hoffmann, A. W., ' Bericht iiber die wissenschaftlichen Apparate aiif der 

 Londoner Internationalen Ausstellung im Jahre 1876.' 8vo, Braunschweig, 1878. 

 Cf. pp. 383-420, Abbe, E., ' Die optischen Hulfsmittel der Mikroskopie.' 



t It appeared to us, on a perusal of Professor Abbe's paper, that he had not 

 gi^en sufficient weight to the increase in aperture and resolving power obtained 

 by the use of homogeneous immersion, but to our objections on that point he 

 writes as follows : — " Your objection leaves out of siglit the general point of view 

 from which the question of further perfection of the Microscope is discussed here. 

 -When the article was written, the opinion was generally spread, still— even 

 among microscopists — that it was only a question of time that the Microscope 

 should display the molecules themselves. This opinion I had always in view during 

 the whole discussion. Hence results the large standard applied by me in 

 estimating and measuring progress. The increase of delineating power from 

 1 • 1 to 1 • 4 or 1 ■ 5 is an exceedingly small increase compared with the supposed 

 increase from 1 • 1 to c». That half the wave-length in air ' is the approximate 

 limit,' and that this will not be overcome in a ' considerable ' extent is true, 

 notwithstanding homogeneous immersion, having regard to the said standard of 

 estimation. The important point is that the wave-length does constitute a limit ; 

 that the value of the wave-length may be reduced in some degree by media of 

 higher refraction is the subordinate feature under the point of view of the paper." 



