2.1. The Measuring Mark 
  
The shape of the measuring mark is illustrated in Fig. 2. It 
consists of an illuminated ring with a concentric light dot 
leaving a free gap between the two. The center part of the 
mark can be switched off, so that a ring remains. Also the 
outer part can be switched off, leaving the inner part as 
measuring mark. It will be clear that through this arrange- 
ment a choice of three marks is obtained which are automati- 
cally concentric so that never any error is introduced by 
switching over to the other measuring mark, because it is 
  
/ 3 
\ 0.16 mm / 
ven 7 
basically the same mark, rigidly connected with the saphire 
needle. The intensity of floating mark and image is adjustable 
for each channel separately. 
For the purpose of pricking three new points at the same time, e e 
any of the three shapes of floating mark can be chosen. 
When a previously pricked point has to be transferred to 
other photography, the ring mark should be chosen to start with, 
which can be conveniently centered on the existing pricked point 
monocularly. After centering, the center part of the floating 
mark is switched on, which will then cover the pricked point so 
that two equal measuring marks appear and the disturbing effect 
Of a pricked point in one of the images is avoided. 
2.2. The Observing System 
  
A binocular observing system can be connected to any two of the three | 
optic channels by means of a mechanical switch. Each channel con- 
tains a dove prism which can be adjusted separately but in addi- | 
tion a device is provided with the help of which all three dove 
prisms can be rotated simultaneously. This simultaneous rotation 
has four definite stops in each full rotation of the image, so 
that Y-parallax can be changed into X-parallax in two positions 
and stereo- as well as pseudo-stereo-obersvation can be used.