ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
273 
R (T - f) 
T, N.A. = — ^ evidentl y f°U° ws from this, that measure- 
ment with the apertometer only gives the N.A. for the particular tube- 
length used in the experiment, and that a lens should not be described as 
of such and such N.A. without mentioning the length of tube with which 
it is intended to be used. 
Of course the deficiency of resolving power with the Continental 
tube-length might be made up for by either increasing the diameter or 
deepening the curves of the lens. But this involves more careful cor- 
rection and increased expense. So that the argument may be put this 
way : the short tube, in order to give the same results in the way of 
resolution as the long, requires to be fitted with a more expensive lens.” 
Optical Theory of the Microscope. The Virtual Image.* — Senhor 
D. Joaquin Ma. de Castellarnau in this little treatise deals with the 
optical theory of the virtual image, a subject which has been somewhat 
neglected in most books on the Microscope. 
The object of an optical instrument is to modify advantageously tho 
retinal image with respect to the limits of visibility. The first section 
of the book is accordingly devoted to the consideration of these limits. 
The function of the eye as an optical instrument, the question of ac- 
commodation, the magnification of the retinal image, and the influence 
of diffraction on its formation, are all fully discussed. The conclusions 
arrived at are that when the eye fails to perceive clearly the form and 
colour of an object it is for the following causes : — The want of sufficient 
illumination as regulated by the aperture of the pupil ; the smallness of 
the retinal image ; the alteration which the image suffers from the effect 
of the diffraction resulting from the pupil ; the fact that the eye does 
not receive and utilize in the formation of the image all the diffracted 
rays resulting from the passage of the light through the small elements 
of the object. 
In the two remaining sections the author shows the effect of the 
simple and compound Microscope in extending these limits of visibility. 
In these sections the subjects of the amplifying power and the numerical 
aperture are fully treated. 
(6) Miscellaneous. 
Exhibition of Microscopes at Antwerp in 1891.+— This exhibition, 
which was chiefly due to the initiative of Dr. H. Van Heurck, was 
opened in the Athenee Eoyal, in Antwerp, on August 9th, 1891. The 
greater part of the exhibits was contained in an immense room, 
along one side of which was arranged a series of old Microscopes, 
illustrating the history and gradual development of the instrument. 
Next to this came the fine collections of Dr. H. Van Heurck, the Micro- 
scopes of Watson and Sons, Zeiss, Powell and Lealand, and Nachet, the 
special collection of Prof. Bolsius, and the bacteriological apparatus of 
Adnet. Down the middle of the room were arranged the Microscopes 
of Reichert, various phases in the construction of objectives of Zeiss, the 
* ‘ Teona Optica del Microscopio. La Irnagen Virtual,’ Cronica Cientifica, Bar- 
celona, 1891, 105 pp. (29 figs.). 
t Ann. dc Mierogr., iv. (1891) pp. 22-30, 69-96, 120-59, 199-219. 
