7otnS.Z™m1 Microscopical Research. 87 
of compound ones, are only employed in transmitted light when 
it is required to ascertain the general forms and relations of the 
tissues. The larger the object is the lower must be the mag- 
nifying power, if it is desired to give a complete view of it. In 
the case of larger objects, however, a general view is taken with 
a low magnifying power, and then the details are studied by going 
over them with a higher power. The very powerful lenses recently 
manufactured by Hartnack serve chiefly for the examination of 
living tissues, or of isolated, well-preserved tissue elements. In 
tissues which have been, for the purposes of investigation, sub- 
mitted to rough treatment — which have been, that is to say, 
hardened by reagents, coloured and repeatedly washed — a high 
magnifying power brings out at first sight little more than those 
of average power; indeed the eye less skilled can see in such 
cases less clearly, when using Hartnack's No. 15 than when using 
Hartnack's No. 8. But high magnifying powers are even here an 
invaluable aid for the beginner when the definition of deep struc- 
tures is required. It is necessary to use the screw with the 
greatest caution and to turn it only very slightly, so as to obtain 
after each slight turn of the screw a new field of vision on which to 
rest, and observe whether to proceed next to a deeper or higher 
one, to advance or withdraw. 
When we have to do with isolated and particularly well-pre- 
served elementary forms ; and further, when the preparations are 
examined when recent and without the addition of fluids, or of 
such only as do not affect them, the greatest advantage is derived 
from the use of high magnifying powers. The advance in our 
knowledge of the cell and of the finer structure of the nerve-fibre 
rests upon investigations with the excellent instruments of modern 
construction. The examinations of the cornea in the living state, 
as they were commenced by Kecklinghausen and Kiihne, must 
convince us more completely of the value of high magnifying 
powers. It is indeed true that the structure of the cornea, when 
fresh, cannot be made out even with the best magnifying powers. 
In its recent condition, only such tissue elements can be distinctly 
seen as refract light otherwise than do the parts surrounding them. 
Thus, if fibres or cells are imbedded in connective substances or 
an interstitial fluid, the optical behaviour of which does not difier 
from that of the tissue elements, then they cannot be seen even 
with the best magnifying powers ; artificial means must be re- 
sorted to. 
These are either mechanical for the purpose of tearing from 
each other the tissue elements, or they are chemical, the use of 
which consists in such cases in either dissolving the cementing 
substances, or changing them diflerenily from the elements them- 
selves. The best artificial preparations, however, cannot replace 
