The following quotations and details are taken from Michelson's description of his study (pages 118-124).
``The study would take place on a clear, almost level, stretch along the north
sea-wall of the Naval Academy. A frame building was erected at the western
end of the line, a plan of which is represented
in Fig. 323
The building was 45 feet long and 14 feet wide, and raised so that the line along which the light travelled was about 11 feet above the ground. A heliostat at H reflected the sun's rays through the slit at S to the revolving mirror R, thence through a hole in the shutter, through the lens, and to the distant mirror.''
The heliostat is an instrument used to focus the sun's rays and direct them in a narrow beam. This then was the source of light. Because it is easier than the heliostat to adjust, a small mirror, F, directs the beam from the heliostat to the slit.
``The lens was mounted in a wooden frame, which was placed on a support moving on a slide, about 16 feet long, placed about 80 feet from the building. ... The fixed mirror was ... about 7 inches in diameter, mounted in a brass frame capable of adjustment in a vertical and horizontal plane by screw motion. .... To facilitate adjustment, a small telescope furnished with cross-hairs was attached to the mirror by a universal joint. The heavy frame was mounted on a brick pier, and the whole surrounded by a wooden case to protect it from the sun.''
Unlike Foucault, a flat mirror was used as the fixed mirror and a lens of long focal length focused the light (an eight inch non-achromatic lens with a 150 foot focus). The lens was placed in position about 80 feet from the building and the fixed mirror a distance of about 1920 feet from the building. Both the mirror M and the lens L needed to be placed perpendicular to a common central axis as in Figure 2.
Michelson gives no account of how the lens came to be positioned but he does describe the positioning of the mirror in some detail. First it was placed in position with the reflective surface facing the hole in the building.
``A theodolite24 was placed at about 100 feet in front of the mirror, and the latter was moved about by the screws till the observer at the theodolite saw the image of his telescope reflected in the center of the mirror. Then the telescope attached to the mirror was pointed (without moving the mirror itself) at a mark on a piece of card-board attached to the theodolite.''
In this way the telescope atop the mirror was placed at right angles to its reflective surface.
``The theodolite was then moved to 1,000 feet, and, if found necessary, the adjustment25 repeated.''
With the telescope thus placed, the mirror was moved until its telescope pointed at the hole in the building. A final adjustment was made by having someone focus a spyglass at the fixed mirror from inside the building. The mirror was then moved using the screws until the observer saw the image of his spyglass reflected centrally in the mirror.
The rotating mirror was a 1.25 inch circular disc (0.2 in. thick) silvered on one side. It was held on a vertical spindle that was in turn held in a cast iron frame. This frame could be tilted side to side and forwards and backwards by means of small cords. The spindle had pointed ends which pivoted in conical sockets in the frame; these were the only contact points between the frame and the spindle. The top part of the spindle passed through the centre of a small wheel inside a circular enclosure attached to the frame. This wheel held the spindle by friction. Forcing air into the enclosure, over the surface of the wheel, and out again in a circular fashion would cause the wheel, and hence the spindle, to turn. The spindle would have to be carefully balanced so that it turned smoothly without wobbling. The air to power this small turbine came from a steam-powered pump located in the basement of the building. A tube connected the pump to the turbine. Because the mirror's rotational speed remains constant only while the pressure from the pump is constant, a system of regulators, valves and feed-back control26 was installed to adjust the pressure and hence the speed.Michelson notes that the system could hold the speed of rotation constant for three or four seconds which was sufficient to make a measurement.
So as to further increase the distance |SR|, the rotating mirror was placed slightly closer to the lens than at the focal point of the lens ( i.e. its parallel beam focus). This would make for a slightly less clear image than having R at the focus as fewer rays strike and are returned from M.
``A limit is soon reached, however, for the quantity of light received diminishes rapidly as the revolving mirror approaches the lens.''
This limit is about 15 feet closer to L than is its focal point. Michelson's previous studies showed that if R rotates at about 258 revolutions per second, and the distance |SR|, or radius,27 is about 28.6 feet, then the deflection should be around 115 mm.