Let us consider an other instrument — the Stereograph SD designed by Prof. F. 
V. Drobyshev and also based on the principle of making the resection with the aid of trans- 
formed ray-bundles. The scheme shown on Fig. 6 represents an aerial photograph, nm, 
obtained with the aid of a lens of a focal length f,, and of a correction plane em,. The linear 
segments nm, and nm have the same abscissa, X. As is seen from the scheme, the points m 
and m, are on one and the same vertical line. The correction à, depends upon the angles ay, 
aq, and segments f, and F. 
We have 
p — 
Af. f, 
and 
o = C0 
AF F 
We have also 
AF xz tga, F 
- = =k (4) 
A fx Y tg 04 I 
or 
(to = ka, (5) 
The correction formula is: 
xx 
09 = f, o (6) 
Consequently, the instrument must include: 
1. picture holders of focal length f, set at the angles of inclination a (o); 
2. correction planes “decentered” at the point of zero warping of the model, and inclined 
at the angles ka, kw; 
3. a binocular system with vertically directed lines of sighting. 
Figure 7 represents a scheme showing all special features of the instrument SD-I; here 
we have: 
— aerial pictures arranged one after 
another along the axis y; 
— the principal carriages of the in- 
strument X, Y,Z; 
— two correction planes which are 
set at the angles ka, kw; 
— resection levers. 
The correction planes change the 
focal length F of the instrument 
while the carriages and the tracing 
pen are being moved in the process of 
compilation of the map. 
In this instrument the resection 
is carried out in a scale which is near 
to that of the corresponding aerial 
photograph; a subsequent enlarge- 
ment of the model scale (up to 2,2) is produced with the aid of a pantograph. We give here 
a photo of the instrument (Fig. 8), from which may be seen its conformity with the design 
scheme as given above.