137 
After the Daguerreotype give place to albumen and col- 
lodion on glass, Martens’ camera was laid aside, but in 1854 
he and a nephew made an effort to replace the bent plate at 
the back of the camera by a flat plate ; and he contrived an 
apparatus for giving such a motion to the plate and lens 
respectively, that he succeeded in getting pictures upon such 
a plate more or less successfully. 
To show what these motions are, we will go back to the 
instrument which has served to illustrate Martens’ first 
camera. We will remove the bent cardboard, and making it 
straight, we will place it against the circular edge which pre- 
viously held it bent. 
If the taper be placed directly opposite the centre of the 
instrument, the flat paper must be at right angles to the ray 
of light, at a tangent to the curve on which the circular 
plate was bent, and the centre of the plate must be opposite 
the taper. 
Fig. 2. 
If the light be shifted to b fig. 3 the lens must be turned, 
and the plate must also be turned so that it is still at right- 
angles to the ray, and tangential to the curve as before, but 
the image must be received away from the centre. This will 
bring us to the position fig. 3 and if the one taper be 
moved in the opposite direction the lens and plate will be as 
shown in fig. 4. 
Fig. 3. 
