Lastly there is the bellows shutter, represented diagrammatically in Fig. rob, and used by many portrait photographers. Two bellows of black opaque cloth are mounted on a light metal frame, somewhat in the form of Japanese lanterns used for illumination. In the position of rest these bellows form a hemisphere, which, when operated by a pneumatic piston, opens from its vertical plane of symmetry, thus having the advantage that, from the point of view of the exposure, the region round which it opens, viz. the zone enclosing the vertical axis of the image, is almost invariably that occupied by the subject. Obviously with this shutter ex tremely short exposures are impossible, but these have only very rarely to be used in the studio.
137. Double-drop Shutters. Although in prin ciple a double-drop shutter, opening and closing from the centre (Mann, 1862), is for use in the diaphragm, numerous models have been made for use before or behind the objective. One of the best of these, although it was first designed years ago (L. R. Decaux, 1893), is frequently used behind the objective (it may, however, also be used as a diaphragm shutter) without any difficulty if of sufficiently large aperture. This is due to the fact that the time during which full light is being admitted nearly always repre sents more than half the total time of operation of the shutter.' In this shutter, which is represented closed and open in Figs. 109 and z ro, the two plates and instead of having rectilinear move ment, are guided by horizontal grooves and and joined by a long member to the end of the arm movable around the pivot M and brought into its position of rest by the spring r. The driving spring R is coiled round the piston rod in the body of the pump P. This
pump is full of air, which the piston, under pressure of the spring R, compresses to the end of the cylinder, where it escapes through very small holes, the exact size of which may be regulated at will by rotation of the button p, which has marked on it a series of numbers corresponding to different exposures.
An extension of the piston rod carries a rack which engages in the toothed wheel Q, which is directly connected with the setting lever and which is fixed to the cam S which forms the vital part in the mechanism. When the key for setting the shutter is pressed, the rack moves towards the right, thus compressing the driving spring and turning the sector S in the opposite way to the hands of a clock. This presses by the tip x on the pin G of the lever L, which is joined at 1W on the anti and which is slightly displaced from its position of rest sufficiently for the sector to pass by, the lever returning quickly to its normal position, the pin G being thenceforth pressed against the sector The triangular pin C of the earn is then caught in the notch e of the bolt D, which pivots round N and is acted on by the spring 1, the shutter thus being set with out any movement of the plates V.
If the bolt D is lowered by pressing on the rod K, the pin c is set free and on being released the spring impresses on the cam a rotation in the opposite direction to that previously described. The lever L is pushed towards the right, but in this direction it engages the arm and consequently moves the plates of the shutter, the aperture of which is fully opened when the cam has turned through about 90, the pin G being pressed against the cam. Before this pin is set free, thus allowing the shutter to close, hs, rvf i-NxTrrl