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Variable Double Episcotister


A brass and steel mechanism is mounted on a sturdy wooden base.

The mechanism consists of two brass wheels, each with two open sectors, which are attached at their centre. Their angular relationship may be changed in order to vary the size of a void space formed by their respective openings while the wheels are spinning.

Behind the wheels is a rotating shaft incorporating a “spiral nut” mechanism. Along with a movable carriage and bearing sleeve, this permits one of the brass wheels to be rotated with respect to the other while the wheels are spinning.

A crank at the rear of the instrument pulls or pushes the carriage and bearing sleeve in order to adjust this rotation.

A pulley at the rear of the rotating shaft provides a point of attachment to an electrical motor which is not included with this instrument.

Holes in the wooden base permit the instrument to be fixed to a surface.

Accession Number: 2016.psy.161

Alternative Name:

Primary Materials: Wood, Brass, Steel


The inside perimeter of the wheel is marked in increments of one. The first ten degrees are marked in .5 degree increments.

Dimensions (cm): Height = 43, Width = 29, Length = 50


An episcotister is a instrument that uses a rapidly spinning disk to produce a semi-transparent field. The disk incorporates an opening of variable size. This may be used for the purposes of optical experimentation to create a measurable reduction in the level of reflected light from a source behind the instrument.

This instrument incorporates an opening whose size may be adjusted while the disk is spinning. This makes it easier for the observer to observe the instrument’s effect by removing the need to stop the wheel in order to adjust the instrument.


Excellect. There is some corrosion and patina on the metal parts as well as minor abrasions to the finish of the wooden frame.

Associated Instruments: 2012.psy.73

Manufacturer: Local U of T workshop

Date of Manufacture: c. 1901 to 1907


This instrument was likely constructed by a University of Toronto workshop for use by Dr. August Kirschmann and his students at the psychological laboratory. It seems to be highly simplified version of the Zimmermann colour variator, an example of which survives in the Department of Psychology collection.

Additional Information and References:

This instrument is a, in certain respects, a simplified version of the Zimmermann Colour Variator.

This instrument, and its related apparatus, may have been referred to on p. 57 of an experimental report entitled: <i> On Colour-Photometry and some Quantitative Relations of the Phenomenon of Purkinje. </i> written by Robert James Wilson. This has been digitized and is available here.

Historical Notes: