Two-circle reflecting goniometer c. 1915-1925 Stoe & Cie Inh., Heidelberg Germany Brass, glass, aluminum, cast iron, steel, lead, Used to measure the face angle between crystal faces 16” high x 17” long x 17” wide HISTORY Goniometers were used in the study of crystallography to measure the angle between the crystal faces. Through the use of goniometers, it was discovered that the crystal face angles are usually constant and represent the mineral from which the crystal is made as each mineral has a unique and fixed face angle. When a crystal’s face angles are known, its mineral content can be determined. Before the development of x-ray crystal structural analysis in the early 20th century, goniometers were a crystallographer’s most important tool. The two-circle goniometer was developed so that multiple face angles could be measured without re-setting the crystal. Before the advent of the two circle goniometer, it was necessary to remove and reset the crystal for each face measurement which was very time consuming. This instrument is about 16” high and 18” at it’s widest point. It weighs approximately 35 pounds and it sits on a sturdy three-legged base. DESCRIPTION This goniometer has four major components and several adjustment features that interact with them. The major components include: 1. An enclosed and beveled 5-1/2” horizontal silvered circle graduated to 1/2 degree and numerically numbered every 10 degrees for the full 360 degrees. The circle is read through a glass covered window with a sunshade, using a fine vernier and inverted image microscope to one minute of arc. There is a clamp and tangent motion with leaf spring return on the circle for fine adjustments. 2. A vertical circle that is also 5-1/2” in diameter and graduated in the same manner as the horizontal circle. It is read by means of a fine vernier and magnifier with a sunshade to one minute of arc. There is a spring loaded adjustable mirror attached to the magnifier presumably to illuminate the graduations if needed. There is a clamp and tangent motion with leaf spring return on the circle for fine adjustments. Attached to the center of the inner-side of the circle (non-graduated side) are the double X-Y adjusters and the centered crystal mounting pin. 3. The collimator is the device used to shine and focus an external light source on the crystal face for the purpose of light reflection into the reading scope. It is mounted on an extension of one of the three base legs and sits about 13” high. It is 6” long x 1-1/2” in diameter. There is a hand-operated aperture wheel in the body of the collimator with six different aperture settings to control the size and shape of the light beam. 4. A crystal face angle reading telescope with two swing away lenses and lenses at the eyepiece and the front of the telescope that pivot up and out of the telescope axis. There are crosshairs at the eyepiece end and focus is achieved by means of a large thumb screw driving a rack and pinion motion which moves the entire telescope closer to or away from the crystal. The 10” long telescope sits on top of a 5” high brass pillar and its overall height is about 13”. FUNCTION The face angles are measured by mounting the crystal in the desired position and shining an external light source through the collimator resulting in a strong narrow beam focused on the crystal face to be measured. The user then observes and centers the beam and crystal face in the telescope. The horizontal and vertical circles are read giving the position of the crystal face. The crystal is then rotated to the next face and the telescope is centered on the beam and face again. The vertical and horizontal circle are then read again giving the angle between the crystal faces. In order to reduce the time consuming act of re-setting the crystal, the two circle goniometer has a significant number of adjustments that can be made to the major components. Collimator adjustments There are six hand-selected apertures and a front lens focused by means of a knurled hand wheel. In addition, the front-mounting bracket has a high and low setting, and the back end can be adjusted up or down approximately 1/2” by means of two adjuster nuts which equates to 1-1/4” tilt of the beam at the crystal. The front-mounting pins are adjustable and will allow the front end to move horizontally about 1/4” or so which equates to a beam adjustment of over 1/2” at the crystal. Reading telescope adjustments The telescope can be moved on the nickel plated brass ring around the perimeter of the instrument. There is an adjustable collar at the top of the brass pillar supporting the scope which allows it to be pivoted horizontally to the left or right. The lenses can be arranged in a way that the telescope has a long-distance focus feature and only the collimator light beam reflected from the crystal face can be seen or they can be positioned to convert the telescope into a low-power microscope for direct crystal observation. The eye piece has four built-in finger actuated slides that control the view area from full size down to zero. Like the collimator, the scope can be tilted at the back end. Vertical circle adjustments The vertical circle rests on a movable bench similar to a lathe bench. It moves into and away from the center of the goniometer by means of a 3” long screw with a built-in bearing. As previously mentioned, there is a double set of XY adjusters. The set closest to the crystal is curved allowing the crystal to be tilted almost 45 degrees in any direction. There are clamp and fine-adjustment motions on both circles. This goniometer is in excellent physical and working condition. The combination of the black, nickel plated brass, and lacquered brass components have created a strikingly beautiful instrument. Circa 1915-1925; it is estimated between 5 to 10 of these were manufactured yearly during this period. Stoe & Cie Inh. Rheinheimer, Heidelberg, Deutschland is inscribed on the horizontal circle.