Prof. Barnard on Daguerreotypes for the Stereoscope. 349 
mirrors, moveable on a common vertical hinge at A. These 
of P being single, the optical axis FA may be directed truly 
toward the hinge A, and the image be formed truly in the middle 
of the screen, at F. Now supposing that it is desired to produce 
two pictures distant from each other (measuring from centre to 
centre) by a space =n, the two mirrors must be carefully moved 
on the hinge A to the positions AM’ and AM”, so that the images 
of P reflected by them shall pass from F to f, and from F' to f’, 
each of these distances being $2. 
In order that the points of view under which these images will 
present P, may be so far different as to correspond to those of the 
two eyes in natural vision, the camera must be placed at a certain ° 
determinate distance from the mirrors. ‘This will be easily as- 
certained without calculation by a person familiar with this pro- 
cess; but it may be found mathematically as follows: 
2. 
two mirrors, and A the hinge. 
Then, the camera being sup- 
posed to be properly# adjusted, 
AF will be the line of its axis, 
and also the direction of the ray 
PA after reflection, while the 
mirrors continue in one plane. 
Let AM’ be the position of one , 
of the mirrors after its displace- 
ment. Then if C be the virtual 
centre of the arrangement of 
lenses, the image of P will be \ 
formed at F’ instead of at F, b 
means of the ray PA’ reflected through C to F’. GG’ the glass 
screen, will of course be perpendicular to the axis AF". 
Draw AB perpendicular to AP, and AB’ perpendicular to AF. 
Put the angular change of position of the mirror M’ (=angle 
MAM’)= «a, the angle ACA’= 6, and the angle APA’= 7. Then 
in the triangle PAB, right-angled at A, angle B=90°—7. It is 
easily seen that BAM= the original angle of incidence of PA. 
Represent this angle by I. 
BAM+MAA‘/=BAA/=I+ « 
Also, as above, ABA’=90° —7 
Whee in the triangle BAA’, the third angle, BA’YA= 90°— 
Be 
é . 
Now, to obtain AA/ in terms of AB, 
sin BA‘A : sin ABA’:: AB: AA’, 
