350 Prof. Barnard on Daguerreotypes for the Stereoscope. 
Or, putting AB= « 
sin (80° -I-a+7) : sin(90°-7) ::4 
Again, in the triangle B/AA’, 
Angle AB/A’, = 90° + 6. 
And, AA’B’ (= AA/B)= 90° —I—« + 7. 
Whence sin AB/A’: sin AA’B’:: AA’: AB’. 
Or sin (90°-+6) : sin (90° -I-a+): 
_ @cos 
* cos (I-+-e-7)’ 
Ooefe: 24g, 
eos (I-+e=7) ‘AP 
,_.acosycos(I4+a—y) acos7 
aod au ~ eos 6 cos (I--a—7) ~ cosé* 
Now AB‘ is parallel to GG: hence, 
pee ab sO <CA; 
which last term is the distance (measured from the virtual centre of 
the objective) at which the camera must be placed from the point A. 
In this proportion, FF’ is arbitrarily fixed, and will be from 1 
to 14 inches, FC is the focal distance of the camera, when the 
image of P is distinct on the screen, and AB/ is determinable by 
the foregoing formula. 
In that formula, « is one half the distance between the eyes 
(14 inches on an average), 6 is directly determinable in the right 
angled triangle CFE”, and ; is in like manner to be obtained from 
the right angled triangle PAB, the distance, AP, of the object 
from A, being ascertained by measurement. : 
he mirrors ought to be such as are prepared for photographic 
purposes ;—that is to say, they should be of the best glass, and 
have their surfaces perfectly parallel, or else they should be of metal. 
Nevertheless I have succeeded very well with good plate looking- 
glass. Isend you a specimen taken with such, that you may 
compare it with specimens otherwise prepared. 'The imperfections 
of the glass will be less sensible, in proportion as the incidence 
approaches perpendicularly ; but there is an obvious limit to this. 
The photographs which I have prepared in this way ate not 
surpassed by any others I have tried. I am accustomed to adjust 
them on the plate at a distance from each other somewhat less 
than that of the eyes (say between 2 and 2} inches from centre 
to centre). 1 employ no optical artifice, (i. e. interposed prisms, 
or lenses excentric to the eyes,) to superpose them ; but looking 
through the centres of the lenses, the superposition takes place 
naturally and easily. If the pictures are rather large, however, 
they must be more widely separated, and then some optical ex- 
pedient must be employed to produce deflection and aid the eye- 
Tuscaloosa, Sept. 6, 1853. 
P. S.—In every daguerreotype for the stereoscope which I have 
seen (as purchased from the opticians) the relief is grossly €X- 
aggerated. You will not find such the case with this. The 
error of the manufacturers has been to make the points of vieW 
(in taking the photographs) too widely different. 
