It will be observed that the glass plate, covered with its film of collodion, was removed directly from the nitrate-of-silver bath to the camera, so as to be exposed to its image while still wet.  It is obvious that this process is one that can hardly be performed conveniently at a distance from the artist’s place of work.  Solutions of nitrate of silver are not carried about and decanted into baths and back again into bottles without tracking their path on persons and things.  The photophobia of the “sensitized” plate, of course, requires a dark apartment of some kind:  commonly a folding tent is made to answer the purpose in photographic excursions.  It becomes, therefore, a serious matter to transport all that is required to make a negative according to the method described.  It has consequently been a great desideratum to find some way of preparing a sensitive plate which could be dried and laid away, retaining its sensitive quality for days or weeks until wanted.  The artist would then have to take with him nothing but his camera and his dry sensitive plates.  After exposing these in the camera, they would be kept in dark boxes until he was ready to develop them at leisure on returning to his atelier.

Many “dry methods” have been contrived, of which the tannin process is in most favor.  The plate, after being “sensitized” and washed, is plunged in a bath containing ten grains of tannin to an ounce of water.  It is then dried, and may be kept for a long time without losing its sensitive quality.  It is placed dry in the camera, and developed by wetting it and then pouring over it a mixture of pyrogallic acid and the solution of nitrate of silver.  Amateurs find this the best way for taking scenery, and produce admirable pictures by it, as we shall mention by-and-by.

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In our former articles we have spoken principally of stereoscopic pictures.  These are still our chief favorites for scenery, for architectural objects, for almost everything but portraits,—­and even these last acquire a reality in the stereoscope which they can get in no other way.  In this third photographic excursion we must only touch briefly upon the stereograph.  Yet we have something to add to what we said before on this topic.

One of the most interesting accessions to our collection is a series of twelve views, on glass, of scenes and objects in California, sent us with unprovoked liberality by the artist, Mr. Watkins.  As specimens of art they are admirable, and some of the subjects are among the most interesting to be found in the whole realm of Nature.  Thus, the great tree, the “Grizzly Giant,” of Mariposa, is shown in two admirable views; the mighty precipice of El Capitan, more than three thousand feet in precipitous height,—­the three conical hill-tops of Yo Semite, taken, not as they soar into the atmosphere, but as they are reflected in the calm waters below,—­these and others are shown, clear, yet soft, vigorous in the foreground, delicately distinct in the distance, in a perfection of art which compares with the finest European work.