Monthly MicroscopicaH 
Journal, Aug. 1, 1869. J 
Microscopical Research. 
85 
the microscopically small space of the instrument and under the 
necessary conditions of its employment, experiments possible which 
would be out of the question with portions of dead tissue. Slight 
changes of temperature, feeble electric currents, and weak acids are 
sufficient to induce changes in living tissues. But if experiment is 
to be made upon dead tissues, then more powerful influences are 
required than the delicate instrument or the observer stooping over 
it are always able to endure. The greater sensitiveness of living 
organisms renders very delicate manipulation necessary, but at the 
same time it facilitates experiment. To this also must it be ascribed 
that only of late years has the latter fact been more largely recog- 
nized, namely, about the time when the investigation of hving tissues 
became more extended. 
The tissues may be examined either by the light which they 
reflect from their surface, or by that which they give passage to — 
indirect or transmitted light. In direct light every object may be 
examined, provided it receives and reflects light enough, and that 
both object and microscope can be fixed. 
It is self-evident that the instrument must admit of being 
focussed, otherwise it will not be possible to obtain well-defined 
retinal images in all the cases coming under examination. Grreat 
enlargement must be dispensed with in direct light, because the 
distance between the objects and the lens must here be small ; so 
that strongly magnifying lenses cover the object and interfere with 
its illumination. It is, however, possible to apply illumination on 
the principle of the ophthalmoscope, and then the difficulty indi- 
cated is overcome. 
The investigation in reflected light gains very much by direct 
illumination, or, what is still better, if a focal image of the source of 
light is thrown upon the place of the object to be examined, details 
will then often appear which can scarcely be observed in difiused 
daylight. 
If longer distances are required in the examination by direct 
light — as, for example, when we work under the microscope with 
larger instruments, or when objects have to be viewed or pre- 
pared under liquids — Briicke's magnifier will be found serviceable. 
This is put into the arm of Nachet's or Hartnack's stand, and the 
object placed upon the stage. Focussing is then efl'ected with the 
unaided hand by moving the magnifier. This combination is of 
great service in the case of delicate preparations with needles, such 
as the isolation of ganglion-cells, or the exhibition of minute fibres. 
There the objects are in every case placed upon an opaque ground 
— on an opaque grey ground if the object is dark ; on an opaque 
black ground if the object is clear. The object to be dissected out 
can, in every case, be placed upon a glass plate, and under this a 
dead white or black slip of paper, as the case requires. For the 
