If on a cold, polished metal, as a new razor, any object, such as a wafer, be laid, and the metal be then breathed upon, and, when the moisture has had time to disappear, the wafer be thrown off, though now the most critical inspection of the polished surface can discover no trace of any form, if we breathe once more upon it, a spectral image of the wafer comes plainly into view; and this may be done again and again.

Nay, more, if the polished metal be carefully put aside where nothing can deteriorate its surface, and be so kept for many months, on breathing again upon it the shadowy form emerges.
Such an illustration shows how trivial an impression may be thus registered and preserved.

But, if, on such an inorganic surface, an impression may thus be indelibly marked, how much more likely in the purposely-constructed ganglion! A shadow never falls upon a wall without leaving thereupon a permanent trace, a trace which might be made visible by resorting to proper processes.

Photographic operations are cases in point.

The portraits of our friends, or landscape views, may be hidden on the sensitive surface from the eye, but they are ready to make their appearance as soon as proper developers are resorted to.