In taking photographs of the sun with the full aperture of this telescope, no driving mechanism is necessary. On the contrary, the difficulty is rather to arrange the apparatus so that an exposure short enough may be given to the sensitive plate, and solarization of the picture avoided. It is not desirable to reduce the aperture, for then the separating power is lessened. The time required to obtain a negative is a very small fraction of a second, for the wavy appearance produced by atmospheric disturbance is not unfrequently observed sharply defined in the photograph, though these aerial motions are so rapid that they can scarcely be counted. Some kind of shutter that can admit and cut off the solar image with great quickness is therefore necessary. [Illustration: Fig. 36. The Spring Shutter.] In front of an ordinary camera _a_, Fig. 36, attached to the eyepiece holder of the telescope, and from which the lenses have been removed, a spring shutter is fixed. It consists of a quadrant of thin wood _b_, fastened by its right angle to one corner of the camera. Over the hole in this quadrant a plate of tin _d_ can be adjusted, and held in position by a screw moving in a slot so as to reduce the hole if desired to a mere slit. It may vary from 1-1/2 inch to less than 1/50 of an inch. The quadrant is drawn downwards by an India-rubber spring _g_, 1 inch wide, 1/8 of an inch thick, and 8 inches long. This spring is stretched when in action to about 12 inches, and when released draws the slit past the aperture _c_ in the camera. Two nicks in the edge of the quadrant serve with the assistance of a pin _e_, which can easily be drawn out by a lever (not shown in the cut), to confine the slit either opposite to or above _c_. A catch at _f_ prevents the shutter recoiling. The sensitive plate is put inside the box as usual in a plate-holder. When a photograph is taken, the spring shutter is drawn up so that the lower nick in the edge of the quadrant is entered by the pin _e_, and the inside of the camera obscured. The front slide of the plateholder is then removed in the usual manner, and the solar image being brought into proper position by the aid of the telescope finder, the trigger retaining _e_ is touched, the shutter flies past _c_, and the sensitive plate may then be removed to be developed.