590 SUMMARY OF CURRENT RESEARCHES RELATING TO 
this result is due as much to the elimination of spherical and chromatic 
aberrations in the new objectives as to the mere increase in numerical 
aperture. The author’s position appears to be that an objective with 
perfect correction will always give truthful images of structures, but that 
one with narrow aperture will only reveal the coarser structure, while 
one with wider aperture will correctly image the finer as well as the 
coarser parts of the structure up to a limit depending on this aperture. 
Limit of Microscopical Vision.* — Dr. A. C. Stokes makes some 
“dogmatic statements” on the limit of the resolving power of the 
Microscope. The greatest number of lines ever seen through a Micro- 
scope is about 120,000 to the inch. The greatest number photographed 
by Van Heurck with the 1/10 in. objective, 1*63 N.A., using mono- 
chromatic blue sunlight, is 127,500 to the inch. This is about the 
number of the longitudinal lines on Amphijpleura pellucida. Dr. A. 
Fock f states that the best Microscopes can resolve, with central 
illumination, 63,750 divisions to the inch ; with oblique illumination 
125,000 to the inch ; and with the aid of photography 3*00,000 ; and 
these limits are probably too high. The theoretical limit is of course 
higher. According to Van Heurck’s calculation a diamond objective, 
2*50 N.A., would theoretically resolve with central illumination and 
white light 120,000 lines to the inch, with blue light 130,000 to the 
inch, and by photography 159,000 to the inch ; while with oblique 
light these numbers would be about doubled. 
(6) Miscellaneous. 
Introduction to the Study of Rocks under the Microscope. £ — This 
text-book “ is intended as a guide to the study of rocks in thin slices ” 
under the Microscope, and, as such, should be of interest to those micro- 
scopists who do not wish to restrict their observations to the organic 
world. 
A short introductory chapter contains a few notes on the optical pro- 
perties of minerals, with instructions on the measurement of extinction 
angles, observation of the interference colours, pleochroism, &c. ; but 
no systematic account is given of the various physical and chemical 
methods for the determination of minerals. For these the reader is 
referred to standard works on the subject. The introduction is there- 
fore followed at once by the systematic description of the various rock 
types, treated for the most part under the three headings : Constituent 
minerals ; Structure ; Leading types. The massive igneous rocks are 
divided by the author into plutonic, intrusive, and volcanic ; and under 
each of these heads the families are taken in order of increasing basicity. 
The sedimentary rocks are dealt with under the tour groups: 
Arenaceous, Argillaceous, Calcareous, and Pyroclastic. This portion 
of the book will supply a real want, since in most previous text-books 
on petrology the massive igneous rocks have alone been considered, 
e. g. we still wait for the second part of Teall’s £ British Petrography.’ 
Finally the metamorphic rocks are treated under the two divisions 
of thermal and dynamic metamorpliism. 
* The Observer, vi. (1895) Pract. Micros., pp. 97-100. 
t See this Journal, 1894, p. 895. 
X A. Harker, ‘ Petrology for Students,’ Cambridge, 1895, 306 pp., 75 figs. 
