ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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optical systems, each acting as a single convergent lens. One, called 
for convenience of description the first lens, occupies the lower part of 
the objective next the object to be examined ; while the other, called 
the second lens, occupies the upper part in connection with the Micro- 
scope-tube. 
Matters are so disposed that the first lens gives behind it and in 
front of the second an inverted image of the object, and the second then 
gives behind it an inverted image of this first. It follows from this 
that this second image, inverted in relation to the first, is really erect in 
respect to the object. As this is the image examined by the eye-piece 
which does not invert, it accordingly remains erect with respect to the 
object. In other words, the aim of the first lens is to give an inverted 
image of the object ; while the second acts as an ordinary objective, and 
with the eye-piece constitutes a compound Microscope; so that we 
examine with this Microscope, not the object itself, but an inverted 
image of it produced by a lens placed in front of the objective, between 
it and the object. The Microscope, as it inverts anew this inverted 
image, gives a final image, which is erect with respect to the object 
examined. 
The possibility of obtaining with these new objectives very long foci, 
and of any length desired, is explained very easily. With convergent 
lenses, the farther the object seen, the nearer to the principal focus is the 
image on the other side of the lens ; so that if it is wished to receive it 
on a screen or examine it with an optical apparatus, it is necessary to 
approach the nearer to the principal focus. Reciprocally, when very 
near the lens, it is only possible to see the images of very distant objects ; 
and, on the other hand, when receding from it, only those of objects 
very near. Similarly, with this new system of objectives, if the second 
lens is brought near the first, only very distant objects can be seen, and 
accordingly the focal length of the whole system will be augmented ; 
while by separating the lenses the focal length will be diminished, and 
only nearer objects can be seen. I have made one of these objectives 
in which the two lenses can be approached or separated at will, so as to 
vary at pleasure the length of the foci, and to see with the Microscope 
objects more or less distant. In practice, however, I think that it is 
better to use objectives with fixed focus. 
The idea of erecting microscopic images by means of the objective is 
not new. Strauss-Durkheim says, in his treatise on Comparative 
Anatomy (I. p. 81), published in 1842, that he succeeded twenty-five 
years previously in erecting the images of compound Microscopes 
by placing an additional objective below the ordinary objective, and he 
describes and figures the arrangement which he adopted. He would seem 
to have shown this improvement to Trecourt and Oberhauser, and it was 
probably their Microscope thus modified which they presented in 1839 
to the Academy of Sciences (ix, p. 322, “ Microscope achromatique a tous 
grossissements ”). It gave very variable magnifications, had foci greater 
as the magnification was weaker, and gave erect images. Fischer de 
Waldheim, of Moscow, had the same idea about the same time, and con- 
structed a Microscope to which he gave the name of pancratic. 
These Microscopes did not appear to have any success. Robin, in 
his ‘ Traite du Microscope’ (1871, p. 1G2), states that their images were 
