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CLASSIFICATION OF INSTRUMENTS, SCHERMERHORN 163 
The consequence of this discussion is that it is desirable to abandon these expres- 
sions of first, second and third-order instruments. 
Our conclusion is that not only the usual classification is nothing else but a rough 
and incomplete method of listing the existing restitution instruments, but that such a 
classification can neither be correct nor complete. With future new instruments we can 
expect that each classification will be even more inacceptable. 
Already now we have instruments which cannot be listed in the textbooks under the 
usual classification. We think of: 
I. Comparators and Stereocomparators 
{I. Plotters with digital realization of the parallax equation or of the double point 
resection and intersection in space 
III. Equipment for planimetric triangulation 
IV. Rectifiers. 
The list will in future become longer which makes the value of each classification 
more doubtful than it is already today. Therefore, describing here the usual classification, 
we know that everybody can succeed in mentioning at least one restitution instrument 
which cannot be brought into one of the classes. 
We must realize that the original and classical distinction between various con- 
structions is due to the difference between the solutions of the problem how to observe 
always a sharp image of all points of the model. To fulfill this condition three different 
classes of instruments have been developed: 
A. Instruments of the photogoniometer type with observation through the lens of the 
'amera by means of a telescope, focussed at infinity, 
B. Instruments with direct optical projection with special devices of precautions to guar- 
antee the required depth of focus in the model, 
C. Instruments with mechanical projection with direct observation of the diapositive by 
some stereoscope. 
Further distinctions can be made using criteria derived from the following elements: 
a. The projection system which represents the bundles of rays as existing during suc- 
cessive exposures. 
b. Devices for scanning of the model and measuring (including registering of coordi- 
nates of points in the model) 
c. The system of observation. 
These subdivisions are as follows: 
A.1. Scanning of the image by rotation of the optical system for observations about two 
axes (Nistri Beta 2) 
A.2. Scanning by rotation of the telescope and of the camera each about one axis (Poi- 
villiers B, Thompson-Watts) 
B.1. With auxiliary optical system for automatic focussing (Zeiss Stereoplanigraph) 
B.2. With reduced size of projectors in order to obtain sufficient depth of focus (Multi- 
plex type) 
With small aperture of special lens of projector for the same purpose (Nistri 
Photomapper, Kelsh Plotter, Kern PG 1) 
C.1. Scanning of the image by movement of the negative and fixed position of the 
optical system (Galileo Santoni, Poivilliers D) 
C.2. Scanning of the image by movement of the optical system connected with the space 
rods, fixed position of the negative (Wild) 
C.3. Scanning of the image by movement of the negative in X-direction and of the 
optical system in Y-direction (Kern PG 2). 
QJ 
B.3.