Rangefinders

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Rangefinders were the primary data-collection tools of a naval action in this period, particularly in the preliminary phases.  The Royal Navy used coincidence rangefinders, primarily, while the Germans relied upon stereoscopic devices.

Much is made of this difference in casual discussion, as it is easily latched onto by casual readers of history.  There does appear to be meaningful differences between coincidence and stereoscopic instruments, but to a certain degree, they rely upon the same principle:  trigonometry.

Human beings, like any animal enjoying binocular vision, gain a measure of depth perception by virtue of each eye seeing the object focused upon from a vantage point 2 inches apart from its neighbor.  There are other visual cues we use in judging ranges, and these also serve as the foundation of yet other methods of mechanical range estimation, but the binocular principle is the common element that unites the systems used in the Great  War.

The Royal Navy's coincidence rangefinders were mostly produced by the Scottish firm of Barr and Stroud, but a number were also produced by the partnership of Cooke-Pollen.  As we have an excellent 3D model of a Cooke-Pollen rangefinder, we will use it to illustrate the principle here.

A nine-foot Cooke-Pollen Coincidence Rangefinder on Gyro-stabilised mounting

Let's leave off for a moment that Rob Brassington has lovingly depicted a stabilized mounting (much of that mechanism is in the housing to the right), and concentrate on the 9 foot cross bar with the seat behind it (facing us).  The primary operator would sit in the seat and look into a pair of eye holes (see below).  Mirrors in the device would take each eye's view, and direct them in opposite directions outward until they were fully 9 feet apart, and then more mirrors would direct the lines of sight forward again through the cylindrical openings you see at the end.  Imagine now that your eyes were 9 feet apart!

View from over operator's shoulder

The rangefinder operated like an old 35mm SLR camera's focusing interface:  the user's two eyes separately supplied the two halves of an image that was split top and bottom, with the left eye (say) seeing the top half from 56 inches to the left and the right eye's view seeing the bottom hemisphere from 56 inches to the right.  By working a knurled thumb knob, the operator would adjust the angle of the outer mirrors and cause them to concentrate his line of vision on the object, and align the top and bottom halves of his binocular viewfinder.  Mechanical equipment would convert the angle of the mirrors which satisfied the operator's sense that he'd aligned the halves perfectly to the range implied (an arctangent relationship), and continually feed this into an electro-mechanical transmitter so it could be sent on to other parts of the ship.

What happened that that juncture differed according to the installation and what means there was to carry the observed range onward to the transmitting station.  In actual practice, the operator would "bless" his current indication by pressing a trigger to signify at the receiver that the presently indicated range appeared correct.  In a ship equipped purely with Pollen-supplied equipment, the intent would have been that the operator at the rangefinder was to continually keep his instrument on the proper range, a feat considerably at odds with the fact that operators could only judge the proper range by going off it and coming back to it, much as a visitor to an optometrist cannot judge perfect focus and is instead asked continually, "is this better, or worse?"

Another image, because we have it.