ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
791 
It will be noted that the formula above given does not tako into 
account the equivalent focus or magnifying power of the lens ; in other 
words, that this factor has no influence on the resolving power of the 
objective. This is correct. A well-connected 1 in. objective will resolve 
exactly as well as an equally good 1/4 in., provided the factors N.A. and 
X remain the same. I here allude to this matter, as every few years 
some one imagines that he is on the verge of great discoveries to bo 
brought about by the simple feat of increasing the amplification of the 
image. Let us assume that a photograph be made with an amplification 
of 3000 diameters, with N.A. the greatest and X the least possible. This 
photograph may then be further enlarged to 30,000 or 300,000, but the 
enlarged pictures will not show any finer or more intimate structure than 
was delineated on the original smaller picture.” * 
(5) Microscopical Optics and Manipulation. 
Unusual Microscopic Images.f — Herr L. Sohncke describes and 
explains a curious observation which he accidentally made with an Abbe 
diffraction plate. He found that, with the Microscope left quite unaltered, 
there were five different distances of the plate at which microscopic 
images of the grating upon it were obtained. The images were partly 
inverted, partly erect, and of different magnifications. The unusual 
images differed from the normal one by considerably diminished bright- 
ness. The phenomenon was not dependent on the particular Microscope 
employed nor on the mode of illumination. It was not connected with 
the known focal properties of diffraction plates, for it was found to be 
quite independent of the grating nature of the object. Its production 
was finally determined to be due to the* plate carrying the object having 
the properties of a mirror. A plate, however, can only act as a mirror 
if it is illuminated from the side of the Microscope. Thus certain faces 
of the Microscope lenses must also act as mirrors. Only bounding 
surfaces between glass and air need be taken into account, not those 
between glass and glass, because in the latter case the reflected intensity 
is too slight. By this double reflection an image of the object may be 
produced at the right object distance from the front lens of the Micro- 
scope. The author shows that in this way the phenomenon in question 
is completely explained. 
The following table contains the data of observation on the position 
and magnitude of the five images which were obtained by a Zeiss Micro- 
* Some months ago a friend, who is a sub-chief in one of the principal bacterio- 
logical laboratories of this city, remarked to the writer that the height of his ambi- 
tion was to possess and work with a Zeiss 1/18 in. apochromatic. I replied that if 
he fancied Zeiss lenses he had better select a 1/12 in., as with it he would be able to 
do more and better work in the line of research and discovery than with the 1/18 in. 
My reply was evidently received with extreme incredulity. If we refer to Zeiss’ 
catalogue, we shall find that the numerical aperture of the 1/18 in. is given as 1 * 18, 
but the 1/12 in. of the same maker has a N.A. of 1 *30. He makes still another 1/12 
in. N.A. 1*10. Applying these figures to the equation above given, and assuming 
for white light \ 5269, we shall find that the 1/18 in. will resolve or differentiate 
particles that approach each other as closely as about 114,000 to the inch, while the 
1/12 in. of N.A. 1’30 will resolve particles as close as about 125,000 to the inch, 
and the 1/12 in., N.A. 1*40, will take optical cognizance of lines or particles that 
approximate each other to within about 1/135,000 in. 
f SB. K. B. Akad. Wiss. Miinchen, 1893, pp. 223-35. 
