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two right-angle prisms, one mounted (on the front of the box) on each side of the lens and so placed as to reflect across the camera front any view of object in front of the camera. Just in front of the lens and in axial line therewith, two smaller prisms were mounted, to reciprocate vertically, the travel being the thickness of one of the prisms. The prisms were crossed, and the reciprocating crank (run at half shutter speed) so timed that the prisms were alternately brought into position before the lens in time with the opening and closing of the shutter. Obviously such an arrangement reflects into the lens a view from first the right prism and then the left prism, so that the negative, when developed, carries alternately right and left eye frames.
The exact distance of the centres of the outside (stationary) prisms is a matter of no particular moment, though 3 inches is convenient and perhaps the best distance. To place them close together exaggerates the size of objects; placing them farther apart makes objects appear smaller, for the base line of the angle photographed is, in the mind when we look at the resultant picture, assumed to be the base line of the angle of vision, that is, the width between the pupils of the eyes.
Doubtless from this thought your mind has run ahead of me questioning, in anticipation, the effect of pictures made from base lines of 25, 50 and 100 feet when viewed in a stereoscope, and what practical application can possibly be made of it.
This question I will answer for you if I find sufficiently valuable pictures I am making from cameras located at the wing tips of a flying machine, and simultaneously exposed. It seems probable that these will be useful in conjunction with map-making cameras because it will give a relative idea of the heights, of known and unknown objects, and if further work proves the scheme has merit I shall be glad to lay the matter before you in detail, perhaps at our next meeting.
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