ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
369 
For cases which require a rigorous exactness the author would keep 
the rule of noting the real magnitude of the object and that of the design 
projected by the camera lucida, but for ordinary purposes proposes a 
simpler method, which consists in placing on the stage the objective 
micrometer, andMrawing its scale on the paper with the camera lucida. 
In this operation the eye must be kept at the same distance from the 
eye-piece, and the paper must be held near the stage and on a level 
with it. 
The operation should be repeated with all the systems in use, and a 
table drawn up giving for each the number obtained by taking the ratio 
between the dimensions of the drawing and the real length of the 
divisions of the micrometer. 
The indications of the table constructed in this way are not rigorously 
exact, but they represent mean values corresponding to the personal 
conditions of the observer, and give the magnification with sufficient 
approximation, so that they are certainly preferable to the pretended 
exactness of the data hitherto in use. 
Resolving Power of the Microscope, and the Future of the Instru- 
ment.* — M. J. Amann discusses the question of the limits to the resolving 
power of the Microscope. The theory of Abbe shows us that the 
formation of the image in the Microscope depends not only on the laws 
of geometrical optics, but also on the more complex phenomena of inter- 
ference and diffraction. The microscopic structure beneath the objective 
determines the formation of a spectrum composed of a maximum corre- 
sponding to the luminous pencil not diffracted and of maxima of the 
second order. In the case of regular structures, the phenomenon is similar 
to that produced by gratings, and especially by gratings traced on trans- 
parent screens, -which Quincke j has studied, and for which he has given 
the formula : 
representing the distribution of the vibratory movement in the plane in 
which the diffractive spectrum is formed. 
When the source of light is very distant, the diffraction spectra are 
formed in the focal plane of the objective. The final image which the 
Microscope gives us of such structures results from the interference of 
the rays emanating irom the spectrum in the particular conditions of 
delimitation of the latter by the aperture of the objective. The mathe- 
matical expression of the repartition of the vibratory movement in the 
plane of the image takes the form of an integral of Fourrier. For the 
Microscope to give us an image of these structures it is necessary and 
sufficient that the objective admit besides the absolute maximum, at least 
one maximum of the second order in the case of a structure similar to a 
* Arch. Sci. Phys. et Nat, xxxiii. (1895) pp. 268-72. 
t Pogg. Ann., cxxxii. p. 361. 
2 B 
1895 
