The 1900 model Nydia shutter was not self-capping, small changes to the tensioning arrangement made around 1902 made it self-capping.

The self-capping model consists of two blades each with a rectangular opening. The blades slide rectilinearly and are attached to an arm which moves in a small arc. Also attached to the arm is a piston fitting into a pneumatic delay cylinder. Around the centre of the arm is a coiled spring the two ends of which are extended to lie each side of a stud on the upper part of the arm.

To tension the shutter one end of the spring is moved by an external lever. When the shutter is released the coiled spring causes the arm to rotate and the blades to slide across. From this position the shutter can again be tensioned by moving the other end of the spring. Thus on alternate exposures each blade moves in the opposite direction, the delay (exposure) is caused by filling or emptying the cylinder of air.

On the earlier, 1900, model one end of the spring is fixed, the other moves with the arm as above. To tension the shutter the arm is moved to the left. The delay is caused by filling the cylinder with air. The blades are the same shape as the later model.

In the illustration of the Self-capping shutter, a) shows the shutter unset. In b) the arm carrying the spring has been moved, by an external lever, to tension the shutter. In c) the shutter is shown after an exposure with the two arms of the spring together again.

In the illustration of the 1900 model the shutter spring and lever are shown tensioned a) and after an exposure b).