Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
§2. Sclerotica and Cornea 
to tenths of a degree. Inside the box, each axle carries a toothed wheel ee 
and a metal frame g in which the glass plate / is fastened. The frame of each 
plate has only three sides, the side turned towards the other glass plate being 
missing. The two glass plates constituted originally a single plane parallel 
Fig. 4. 
plate. A complete metal frame was made for this plate and fastened between 
the surfaces of the two toothed wheels. Then the axles were rounded off on a 
lathe and finally the frame was cut through in the middle. The glass plate 
was cut through in the same manner, and each half fastened in its correspond¬ 
ing half of the frame. Thus an exact adjustment of the positions of the plates 
on the two axles was accomplished. The toothed wheels are moved by the 
sprockets C\ and c2 which are rigidly connected with the axles biCi and &2c2. 
Moreover, each of these axles has a 
sprocket h in the middle. If the 
knob at b\ is turned, the lower 
toothed wheel controlling the lower 
glass plate will be moved by means 
of the sprocket C\. Furthermore, 
the sprocket hi engages with the 
sprocket h2 and rotates the second 
axle b2c2 by an equal amount in the 
opposite direction. As a result of 
this the sprocket c2 acts upon the 
upper toothed wheel connected with 
the upper glass plate and turns it 
through an angle almost as large 
Fig. 5. 
__ w w that of the lower plate. The rota¬ 
tion of each plate is measured by the graduated discs attached to the axles 
on the outside of the box. 


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