H. J. Clark on the = 43 
do not mean to say that a certain amplification obtained by a 
low objective and a high orthoscopic ocular is fully as good as 
the same afforded by a higher objective; but in case the latter 
cannot reach a certain internal structure, the former can be us 
with very little appreciable ge and is by far better than 
the usual methods employed in such cases, such as “eaelg or 
dissections and the isolation of ihe. organ to be i investigated. 
have not had an opportunity to make frequent use of the 
‘‘orthoscopie eye-piece ;” but Mr. Spencer has furnished me with 
another form of ocular, 7 ‘solid eye-piece,” invented by his 
pupil, Mr. Tolls. This, ar SU tells me, so closely ap- 
proaches the “orthoscopic eye- piece” wb rs that none but 
ifference, and the for- 
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objective I have run the nifying power up to two thousan 
diameters, with wonderful Senate which fully justify me in say- 
ing all that I have in regard to the study of thick tissues with 
low powers having bers angles of aperture. 
I will take a young fish as an example to illustrate the re- 
_ markable efficiency of the flat field. In a view from above, one 
_ may see no less than six or seven differe ge or sets 
organs resting one over the other; first the Bs and the muscu- 
Jar layer, next the vertebra, within these the spinal marrow, and 
below the latter the chorda. dorsalis, and close to this the do 
7 artery, then the intestines and their appanage 4 re yet every 
_ one of these may be plunged through and totally ignored, on 
4, account of the peculiar properties of the flat fell 5 the last, 
~ the intestines, minutely inspected, not only cell by cell, but each 
_ cell may be studied, in every particular of detail, as if it were 
isolated. And so may any set of organs be treated, whether 
q situated above or below in the animal. With such means at 
4 itis utterly preposterous to trust what may be worked. out by 
- separating these organs from the animal, piecemeal. When in- 
~— a cell may be measured, not only transversely, but also 
the greatest nicety in a perpendicular direction, by the 
* In this connection I would urge upon students s the necessity of avoiding the 
| 
higher po im the commencement of their studies. When 
they have learned to use the lower objectives, it will be a m ier matter to 
he ones. Students usuall _that they can see everything 
ge the higher powers, whereas they are greatly mistaken ; as much as would 
who should make a minute inspection of the stones of some great architectural 
5 tie a obtained a proper conception of its magnificent plan 
a ome and then think | 
and glorious proportions, 
