excites it. We may, perhaps, conclude from the above experiment (16) that the unexcited retina, if left to itself, is drawn together, and occupies a less space than in its active state, produced by the excitement of light.
Hence Kepler says very beautifully: "Certum est vel in retinâ caussâ picturæ, vel in spiritibus caussâ impressionis, exsistere dilatationem lucidorum."—Paralip. in Vitellionem, p. 220. Scherfer expresses a similar conjecture.—Note A.
19.
However this may be, both impressions derived from such objects remain in the organ itself,