348 Prof. Barnard on Daguerreotypes for the Stereoscope. 
nitric acid ; in the other liquids used in the trials, positive elec- 
tricity passes to the iron instead of proceeding from it. In the 
potash liquid, the reactions have been the same both for the active 
and passive metals, which seems to show that the passivity of 
these related metals is destroyed by this alkali; in fact, if after 
this immersion in potash, they are put in contact with nitric acid 
of density 1-34, they are found to have resumed the electro-chem- 
ical condition which belongs to the passive state and they are no 
longer attacked by the acid. 
The following series exhibits the relations respectively of iron, 
nickel, and cobalt to the two states, commencing - with the posi- 
tive metal, and ending with the negative ; the experiments were 
made with the different liquids mentioned in the table : 
Liquids employed. Active Metals. | Passive Metals. 
Fuming nitric acid. ee Sas + Co, Ni, Fe, — 
et: - ceigasl: Fe, Co, Ni. | Go; Ni, Fe. 
SH Co, Fe, Ni Ni, Co, Fe. 
SH, with 9H. Fe, Ni, Co, Fe, Co, Ni. 
Potash solution. Fe, Ni, Co. | Fe, Ni, Co. 
Palladium is equally susceptible to the passive state. 
Arr. XXXV.—Method of taking Daguerreotype Pictures for 
the Siereoscope, simultaneously, upon the same plate, with an 
ordinar ; . A. P. Barnarp, Professor of Chem- 
istry and Natural History, in the University of Alabama. 
Prof. Dana.—In the September number of the Journal of Sci- 
ence, just published, I observe a mention of a method of taking 
photographic pictures for the stereoscope, the two pictures being 
taken simultaneously. This has brought to my mind an arrange- 
ment employed by myself about a year ago for a similar purpose, 
which is so simple and satisfactory in its results, that you may 
ing two object-glasses, (without, at least, a very inconvenient al- 
rangement of mirrors,) because, of the two pictures produced 10 
such a camera upon one plate, the right hand one will be that 
which should belong to the left eye, and vice versa. 
Fig. 1 is a plan of the k ; 
arrangement which I haye — »st 
employed. C is the camera, eae Let P 
-Pacentral point in an object 
to be copied, and AM, A 
ig “SS ; 
two small vertical plane sd y ‘* 
