38 Bird - Lore The fault of this device was that light came in through the ground glass at the top while the mirror was raising and fogged the plate. To avoid this, a red ground glass was put in, but, in fig. i. interior of right side of mirror camera (hood closed) focusing, the image appeared so faintly upon it that I concluded this would not answer. I finally hit upon the focal plane shutter, and this solved the difficulty. There is nothing new about the focal plane, or curtain shutter ; it has been in use for years ; but the one I have employed differs from the one made by the Thornton-Pickard people in that the spring is not coiled in the roller, on the plan of a Hartshorn shade roller, but is on the side of the box, where it is attached to double cogs, as shown in Fig. i, A. This is stronger than the coiled spring, and gives a more uniform tension and consequently a more even exposure. The focal plane shutter may be used in front of or immediately behind the lens, or just in front of the plate. In the camera here described, as may be seen by reference to the illustration (Fig. i, B, B), the curtain works just in advance of the plate and as close to it as possible. The curtain is provided with an adjustable slot (Fig. I, C to C), which may be widened or narrowed down as circum- stances require. The slot forms the speeder, for obviously the nar- rower the slot the less the time of exposure while the slot is traveling down across the plate, and vice versa. The curtain works upon two rollers (Fig. i, D and E). To make the exposure the curtain is wound up on the upper roller, E, by turning a key fast to the roller, E, and projecting through to the outside of the box. As the curtain is rolled upon the upper roller the tension is increased upon the clock spring attached to the large cogwheel
