252 General Notes. [ April, 
; MICROSCOPY.! 
Mope or Propuction or MicroscoricaL Imaces. — Professor 
Abbe, of Jena, has lately’ established a conception of the manner by 
which images are produced in the microscope, which is entirely dif- 
ferent from those usually adopted. The microscopical image of the 
object is formed by the superposition of two images, which have an en- 
tirely different origin, and can in fact be conceived to be separated one 
from the other. One image is a negative one, by which all parts are 
represented as a geometrical likeness by the unequal emersion of the 
rays of light passing through the object. This image is called by Abbe 
“absorption image.” It represents the definition of the microscope. 
The other image (formed by as many partial images as there are 
bundles of rays which have been isolated from the cone of light, and pass 
into the object) is positive. It is an image produced by refraction, and 
represents the penetration, that is, the finer structure of the object. 
erever the structural elements of the object are small enough and 
approximated enough, phenomena of diffraction appear. The conse- 
quence is that structural images, produced by a coöperation of the frac- 
tion of the rays of light, are not in a constant connection with the real 
structure of the object which produced it, but in constant connection with 
the phenomenon of diffraction which brought about the image. 3 
Microscopical images, therefore, showing systems of fine lines, as in 
diatoms, do not allow us to infer with safety the morphological existence 
of such structures, but only the existence of structures necessary to bring 
about such images. Consequently, the smaller the linear dimensions of 
a structure, the more unsafe are the conclusions respecting the real struct- 
ure as indicated by the image. It can therefore never be decided with 
certainty by what sort of structure the systems of lines (as for instance 
those of Pleurosigma angulatum) are produced, nor will the image of 
the finer transverse lines of muscular fibres give certain conclusions 
about the arrangement of the finer details of structure. This want of 
certainty may also apply to differences in the degree of transparence of 
objects, their color and polarization. 
bbe’s researches allow us to limit with certainty the powers of the 
microscope. “ Never can parts be seen which are so nearly approxi- 
mated that even the first bundles of rays of light produced by fraction 
are not able to enter the objective at the same time as the unbroken cone 
of light.” Every aperture of the objective has a fixed limit for the 
smallest distance of objects by which it is possible to see the object- 
Any new perfection of the microscope cannot go much further than 
to show for central illumination the whole length of. one wave of blue 
light, and for the greatest possible oblique illumination half the length 
of a wave. 
t This department is conducted by Dr. R. H. Warp, Troy, N. Y. 
2 Archiv fiir mikroskopische Anatomie, 1873, ix. 413-468. 
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