164 CLASSIFICATION OF INSTRUMENTS, SCHERMERHORN 
In addition to these three categories, there is at present a particular class which, 
from a certain viewpoint, belongs to one of the classes, but which has a special character. 
This is: 
D. Instruments with affine deformation of the bundle of rays. 
To this class belong: 
D.1. Restitution of super wide angle and multilens photographs in wide angle multiplex 
(Photogrammetrie, München) (of the B2 family) 
D.2. Instruments with mechanical projection and a fixed principal distance, k times 
that of the photographie camera (Romanofsky Stereoprojektor SPR 2, Drobyshew 
Stereograph). 
For so-called third order instruments we may remark that the usual popular classi- 
fication coincides with the scientific classification indicated above. The category so 
classified contains all those instruments in which a correct solution cannot be realized, 
but in which some approximation is inherent, however different these approximations 
may be. We will not repeat here the distinction between the various types of these 
instruments which can be found in the article of H. C. Zorn. See Photogr., XVI, 3, p. 170. 
Our final conclusion remains as mentioned above. Each effort for classification 
remains hopeless. I am sure that notwithstanding the fact that neither the C-factor nor 
the expressions “first and second order plotters” are correct, many photogrammetrists, 
in particular in conversations, will continue to use these expressions. The sense of this 
short article therefore is only to stimulate thought about this subject and to restrain 
authors from the use of these misleading expressions in their publications. 
Is there no other possibility to use simple characterizations of a type of instrument? 
Perhaps, if we do not wish to use simplifications of the scientific classification as a 
kind of complete classification. We could perhaps consider the possibility of using some 
expressions which indicate the striking characteristic of a certain type. This could be 
obtained by talking about: 
1. universal plotters 
2. anaglyphic plotters 
3. affine projection plotters 
4. super wide-angle projection plotters 
5. mechanical projection plotters 
6. digital plotters, etc. 
It is obvious that these indications are not intended to separate the groups because 
for instance the anaglyphic super wide-angle multiplex belongs to groups 2 and 4, the 
Wild instruments A9, B8 and B9 to 4 and 5. This, however, is not a great disadvantage 
since it depends which quality in a certain relation should be stressed most. 
I regret very much that the result of these considerations is rather negative. Some- 
thing more positive is felt not only to be unjustified, but may also contain a danger of 
confused thinking among photogrammetrists. 
RESUME. 
L'article expose qu'une classification des instruments en lier, 2me et 3me ordre, dans 
laquelle la précision est apparemment la marque distinctive, est inutilisable. Depuis que 
les instruments mécaniques, eux aussi, ont été munis d'un équipement pour la restitution 
des vues convergentes, les instruments, dans lesquels on peut exécuter une triangulation 
avec base intérieure et extérieure, pourraient être indiqués comme instruments universels. 
Le plus difficile c’est le groupe étendu des instruments qui, jusqu’à ce jour, est in- 
diqué par ,,2me ordre". Pour le groupe, ayant à peu prés la méme précision que les in- 
  
   
   
    
   
  
  
  
  
  
     
   
     
   
  
  
  
  
  
  
   
   
    
  
   
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
    
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