Version 1 (custom machined, estimated cost $1500)
Front View, showing mirrors, eyepieces, biteboard, screw adjustments for IPD and short handles for adjustment of mirror angle (side mirrors). Looking through the two eye pieces, you're seeing a a pair of front-surface mirrors that reflect to the eyes whatever a second set of front surface mirrors (located at either side of the central unit) is aimed at (usually a video screen)
Close-up view of front of eyepiece, showing mirrors angled +/- 45 from the line of sight. These two front surface mirrors are fixed in place. Two more front surfaces mirrors are mounted at the ends of the eyepiece, one at the left and the other at the right (in this photo they appear as grey squares). The angle formed by those mirrors is adjustable (by turning the aluminum block on which they're mounted - see small "handle" on the left of the left-hand mirror).
View from the "back" of the stereoscope (the observer's head would be on the other side). The two mirrors are the ones glued to the aluminum blocks, whose angle is adjustable using the round "handles" which can be seen in this photo. The grey square in this photo is the back of the aluminum block to which the other pair of mirrors is glued. You can see the lens holders reflected in the mirrors.
For your amusement, here's a photo of a naive observer looking through the device:
Version 2 (commercially available components, estimated cost $1075)
The headrest and holder for the optical rail are available from:
http://www.opt.uh.edu/uhcotech/headspot.html
The optical components are available from:
(4) Mirror Mount 112-0255
(4) Mirror Holder 112-0266
(4) Round Mirrors 033-0290
(4) Optical Carrier 146-1335
(1) Optical Rail 146-0390
Current prices and detailed descriptions of the components are given on those websites. Below are three jpegs showing the headrest and optical rail (lefthand image), a closeup of the optics for achieving dichoptic stimulation (middle image) and the entire set-up with the optical components mounted on the rail (righthand image). The positions of the mirrors can be adjusted to achieve stable alignment of the two half-images; ad justments are made using small, precision screws that change the angles of the mirror mounts.
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