•72 O B S E R V A T I O N S O N ' T H E
I found that tallow, or any other foft fubftance, had the fame appearance,
after being fpread, with a knife, upon a pkite of glafs, as when, after being
melted, it was allowed to cool upon the glafs. Yet although, after
being melted and cooled upon the glafs, it had iliot into ferpentine fibres,
in the fame manner as falts fhoot into cryftals, the figures of thefe lliould
have been deftroyed, or altered, by the mechanical preifure of the knife.
4. In like manner, when I tried a fimilar experiment, fuggefted by my
learned and judicious friend Dr Rutherford, of comparing gold, beaten into
exceedingly thin leaves, with gold melted and cooled again, I found no
diffex'ence in theii" appearance; whereas, if the gold i-cally confifted of ferpentine
fibres, thefe, by the beating, fhould have become broader, and,
of courfe, iliould have appeared larger, and lefs numerous in the fame
fpace.
As thefe appearances produced by the microfcope are fo extremely regular
and diftind, as to be veiy apt to miflead; as they have efcaped the obfervation
of authors ; and, as the confideration of the caufes of them may
tend to throw fome light on the fcicnce of optics, I thought it might be
acceptable to the reader to fee delineations of them. I have, therefore,
added tables reprefenting different objedls, viewed with the microfcope
T. II.
IN material anatomical points, I fufpeil that this deception, produced
by the microfcope, has miflcd fome authors, particularly the late Mr Hewfon
and Mr Falconer. In Mr Falconer's Experimental Inquiries, Plate iv.
fig. 4. cells are delineated, fuch as are faid to be obferved in the lymphatic
glands, which cells will, I apprehend, be found to exift in the microfcopc
only, this figure being merely an imperfea: reprefentation of the deception
produced by that inftrument.
The
• See Tab. XXXV. to Tab. XLV. inclufive.
N E R V O U S S Y S T E M . n
* The fize and fliape of cells in the fpleen,' we are told by Mr Falconer^
' fo nearly refemble thofe we have before defcribedih the lymphatic glands,
' that a tolerably accurate idea of them may be obtained by referring to
• the above mentioned plate *.'
I would here repeat my obfervation made on the fuppofed cells of thé
lymphatic glands, adding to it, as a decifive pi'oof that a miftake was cdmmitted
by thefe authors, either in fa£l, or in theory, about the fpleen, that
the diameter of thefe cells reprefented, is not larger than the fixtieth part
of an inch, when viewed through a lens of yVth of an inch focus, fo that
their real diameter iliould not exceed the 24,000th part of an inch, yet the
red particles, the diameter of one of which is faid, by Mr Hewfon, to be
almoft equal to the -rrrrth part of an inch, are by them fuppofed to be
contained and completed within thefe cells.
Nay, in Plate iv. fig. 2. the veficles of the red particles of the blood,
viewed through a lens of »Vd of an inch focus, are delineated nearly double
the diameter of the cells of the fpleen, though viewed by a lens rVth of an
inch focus, reprefented in Fig. 4. ; fo that one veficle of the blood is reprefented
by Mr Falconer himfelf above fixty times the fize of one of the cells
of the fpleen, within which, he teaches, it was contained and formed.
T C H A P-
• Falconer, Exper. Inq. Seft. 65. p. 108.