An original stereoscope design 
Kalevi Eranti, Helsinki 
Abstract: 
The disadvantage of the conventional mirror stereoscope — that only one stereo-pair can 
be observed at a time — led to the concept of a new type of stereoscope which could be 
used to scan a properly oriented strip of photographs. The new stereoscope is described 
and some applications discussed. 
In the planning of cities, highways, railways, electricity transmission lines and pipe lines 
etc. a map is needed as the planning medium. Even the best of maps, however, does not 
give the planner a vivid picture of the country. I might go so far as to say that the planner 
has a burning need for a living picture of the country. The optical stereomodel formed by 
a stereopair of photographs is a good solution. A good example is the popularity of the 
Balplex — in spite of its limitations — among planners, for example in Finland and 
Sweden. Planners who have not used, for example, the mirror stereoscope will, in general 
be enthusiastic about its possibilities. But the inital enthusiasm wears off very quickly 
since spatial observation is only possible for one model at a time and this creates a 
troublesome problem. It has until now been impossible to obtain a general stereoscopic 
image of the entire terrain although a single stereomodel is still an important aid in 
planning. These thoughts, arising during practical activities, gave me the inspiration to 
pursue the idea further and then suddenly I found the solution for the continuous 
spatial observation of more than two aerial photographs. I was surprised at the extra- 
ordinary simplicity of the solution.