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PolemoscopeWe are proud to post our very first Curious Finds entry, where the Reader Services team liaise with the readers to present something interesting found in our collections. We hope you find these Curious Finds as compelling as we do.

This Curious Find was brought to our attention via one of our regular readers, Mr. Michael Gilligan. Mr. Gilligan is studying the history of science and this find represents his interest in optical devices.

It can be said that we currently live in an age of surveillance. We take the intrusion of CCTV into our lives for granted for example, and the way we look at one another has fundamentally been changed by digital cameras.

In the past as today we used devices to bend images to suit our needs, and one such instrument was the opera glass. The opera glass is the low-power Galilean binocular device used in theatres to enlarge a far away view, however in A New and Compendious System of Optics Benjamin Martin presents a different description of opera glass, one with a much more furtive function.

Martin writes that this version of the opera glass, or Diagonal Perspective, was a device often used by viewers in play-houses, although this device was used for more voyeuristic purposes.

As can be seen in the illustration above from A New and Compendious System of Optics in this post, this opera glass was a wooden tube about four inches long with two holes in the side and a plane mirror inserted which reflected the rays falling upon it to the convex, through which they are refracted to the concave eye-glass, where they emerge parallel to the eye at the hole in the end of the tube. The device is essentially the Galilean binocular device with an end piece containing an angled mirror.

Martin goes on to describe how the function of this device was to view a person at a small distance in such a manner that they would not realise that they were being looked at, as the device would be looked through parallel to the viewed person instead of directly at them:

“…And by which she could take a view of any person she pleased without his having the least suspicion of it, as the Glass was directed quite another way.”

In The General Magazine of Arts and Sciences, Philosophical, Philological, Mathematical, and Mechanical Benjamin Martin goes on to identify this device as a Polemoscope (the name signifying a polemic or contrary view) and indicates its military application for the use of spying:

“….They have often occasion to see what is doing by persons on either side, above, or below the place where they stand, and which, with this contrivance, they can easily command a view of; for, as you turn the glass in your hand, you take in all objects in a lateral situation, or such as are placed side-ways, and cannot be seen by a common telescope. If you turn the speculum downward, you see all below; if you turn it upwards, the top of the buildings, and the people there, are exposed to your view: And thus the actions of the adverse party may be reconnoitred, while they themselves are quite secure from your notice.”

The principle of this design is also utilised in microscopes or for taking close-up photographs. In this instance a light would be shone through the side hole of the device, which would bounce off the pane of glass inside and illuminate the object being observed.

And so the next time you’re at the theatre and using those old fashioned binoculars to see the stage more clearly, spare a thought for the more voyeuristic function of these devices.

If anyone recognises any literary references to this device we would love for your finds or any other opinions to be posted in the comment section.