; aM Jour, Scr—szc 
N > em ™ ae 
O. N. Rood on producing Stereographs by Han 1 yi 
each time that depth is to be represented. This in practice is a 
very simple operation, and if the displacements toward the right 
hand have been rightly proportioned the effect produced is good.* 
The process is also a rapid one, for it consists only in making 
two tracings. 
The question now arises as to the total amount of displace- 
ment allowable, and as to its proper distribution. 
If a number of stereoscopic photographs are examined it will 
be found that in those where the sum of all the displacements is 
not more than one or two tenths of an inch the two images are 
readily united, and a good general effect produced, while in 
those where this quantity is as great as three or four tenths, diffi- 
culty is experienced in effecting the union. 
Taking then -2 of an inch as the maximum total displacement 
allowable, the method of its distribution from foreground to 
ackground can be calculated in the following way : 
‘ Let EE’ be the distance between the eyes 
: =2°5 in., QD=6 in. the nearest distance of dis- 
tinct vision; CD=1 in. A small object being 
placed at D and another at C, the eye EH sees 
.g the two objects projected in the line FG ate 
and D; the eye H’ sees the same objects at D 
andc’. Here the total displacement, oe 
the picture seen by the left eye as a standd 
of comparison, is equal to the distance ec’, 
With the proportion :— ; 
QC : HQ=(1'25 in.) :: CD: cD= < 
we have at once the value of this quantity. In like manner 
this displacement on the line FG for 2, 3, 4 or any number of 
inches from Q is readily calculated. | 
As an example I subjoin the following table applicable to 
~ 
small models: 
For the 19th inch above 6 inches, 
Total displacement. 
or for the 25th inch, . 1-900 inch, eo 2s 
i ae ‘ «9022 
te ee ee es * +040 
ee ee os POU * -060 
id a ee 1980.“ “ +080 
ih 8 42 000. “* +100 
|. hice 2016 “ 4. ie 
se bin OO ‘¢_o. 
os eee 2045 « ‘ 145 
OR ak ek a ORE «  e. 
oN LU a wee 4) Be 
36 Ci 
: imes ha 
Th hidden in the left picture 
ee hu a 
