30 ELEMENTARY PRINCIPLES OF' MICROSCOPICAL OPTICS 



angle. Dr. Royston-Pigott constructed the first aperture table 

 giving the relative values of dry, water, and homogeneous (nascent 

 pencil) immersion objectives ; it is given in M. M. J., vol. iv. p. 26, 

 (1870). 



One of the essential advantages of this system, beyond those 

 stated, is that by the suppression of spherical aberration in front of 

 the objective, facilities are afforded for correcting objectives of great 

 numerical aperture, both in theory and practice, that reduce it to 

 the level of the problem of correcting objectives of moderate ' angle.' 

 As a result, stimulated by the manifest advantage to be obtained 

 and the wants of those engaged in actual research, Messrs. Powell & 

 Lealand, of London, very soon made a .r-th i ncft an d a sV* n i nc ^ 

 objective on the homogeneous principle, with numerical apertures 

 respectively of 1*38, and during the year 1885 produced lenses of an 

 excellence impossible to any previous system of -^th inch, T Vfch inch, 

 and -gVkh i ncn po wer ? having respectively numerical apertures of 

 1-50, while 1'52 is the theoretical maximum. 



The use of a ' correction collar ' in homogeneous object-glassee 

 has been dispensed with, correction being obtained by alteration of 

 the tube length solely, but this must also be aided in endeavour- 

 ing to secure the most perfect ' critical images ' by a body-tube pro- 

 vided with rack and pinion motion ; this should be of the best 

 quality, and if the object-glass is of perfect construction and of latest 

 form (apochromatic, q.v.), results never before attainable can be got 

 with comparative ease. 



With such evidence of advance in the optical construction of 

 microscopes, dependent apparently on such accessible conditions, the 

 question of what is possible in the future of the instrument no doubt 

 obtrudes itself; that, however, can only be considered as having 

 application to the area of our present knowledge and resources. It 

 is impossible to forecast the future agencies which may be at the 

 disposal of the practical optician. To photograph stars in the im- 

 measurable amplitudes of space, absolutely invisible to the human 

 eye, however aided, was hardly within the purview of the astronomers 

 of a quarter of a century ago ; that there may be energies and 

 methods discoverable by man that will open up possibilities to the 

 eager student of the minute in nature which will just as widely 

 overstep our present methods of optical demonstration, there can be 

 little reason to question. But it is no doubt true that with the in- 

 struments and media now at the disposal of the practical optician 

 no indefinite and startling advance in microscopic optics is to be 

 looked for. The * atom ' is infinitely inaccessible with any conceiv- 

 able application of all the resources within our reach. But optical 

 improvement of great value, bringing nature more and more nearly 

 and accurately within our ken and reducing more and more certainly 

 the interpretation of the most difficult textures and constructions 

 in the minutest accessible tissue to an exact method, is certainly 

 within our sight and reach. It is not a small matter that the homo- 

 geneous lenses were, in a comparatively short period of time, carried 

 from a N.A. of 1'25 to 1'50 ; and this carried with it the capacity 

 theoretically indicated. 



