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SUMMARY OF CURRENT RESEARCHES RELATING TO 
is so slight as to make small animals transparent, a very obvious pheno- 
menon in the case of rounded pelagic animals, freshwater or marine. It 
is only by slight differences of refractivity, often requiring accentuation 
by oblique illumination, that we show up the different structures and 
cavities in these, in optical sections under the Microscope. Exner has 
recently determined the refractivity of striated muscle in Hydrophilus 
at p = 1*363, in the frog at p = 1*368,* while that of water is 
p = 1*333, that of the liquid of an ovarian cyst is p = 1*365, synovia 
p = 1 * 348, egg-albumen (fresh) p 1 * 359-1 * 364. j 
Immediately on death, however, this transparency disappears, and dead 
protoplasm is notably opaque. Now opacity can only be due to one of 
three causes : reflection at the surface, as with metals ; absorption in the 
substance, as with ink ; or scattering of light due to optical hetero- 
geneity, like spun glass in the skein, silicate wool, filter-paper, &c. The 
two former causes are excluded by the nature of the case ; and the last, 
optical heterogeneity , is left to us as the only possible explanation. Now 
we can make the paper transparent by greasing it, and so replacing the 
air in its interspaces by a medium approaching cellulose in refractivity ; 
just so do we “ clear ” or restore the transparency of our dead proto- 
plasm by replacing the aqueous medium that permeates it (and which 
can be expelled by pressure), with one of higher refractive index such 
as glycerin (p = 1*462), or Canada balsam ( p = 1*52)4 Yet even the 
latter falls below that of the dead protoplasm, as is very obvious in 
balsam mounts of transverse sections of muscle, or the spores of Sapro- 
legnieae, and I think their index is not much under 1*55. 
It is obvious that the watery liquid which permeates dead protoplasm 
(consisting of water -[- small quantities of soluble salts) exists in a 
separate liquid condition in the interstices of a solid material, and that 
these interstices are too fine to be directly visible by the highest powers 
of the Microscope. This solid material has lost the power which it 
possesses in life of taking water into its substance. Exner has shown 
that living muscular fibre can excrete part of its liquid with corre- 
sponding increase of refractivity, and he cites the observation of Kiinckel 
that it can take up an additional 20 per cent, of water. 
It follows that the water in living protoplasm must exist in a state 
of perfect physical combination, like the water of a solution of gum or 
of jelly. Now the phenomena of protoplasmic motions, as studied in the 
Rhizopods and in the vegetable cell, seem to me absolutely to preclude 
the jelly supposition ; and for these cases we must admit that living 
protoplasm is a viscid liquid, whose refractivity is probably the mean 
of the two constituents separated by death, the one solid, the other a 
watery solution ; and death is for us essentially a phenomenon of 
precipitation. 
I may summarize these conclusions in the following theses : — 
I. Live protoplasm is transparent and of low refractivity (/x< 1*38). 
II. Dead protoplasm is opaque from optical heterogeneity. 
III. The transparency of dead protoplasm is restored by replacing 
the liquid that permeates it by a medium of higher refractive index. 
* In Pfliiger’s Arch., xl., “ Ueber optische Eigenschaften lebender Muskelfasern.” 
f Exner in Arch. f. Mikr. Anat., xxv., “ Ein Mikro-Refractometer,” p. 111. 
X This explanation of clearing was first given in part by A. B. Lee (‘ Micro- 
tomist’s Vade-Mecum,’ 1st ed., p. 213). 
