Babinet goniometer c. 4th quarter 19th century Ed Lutz, Paris Brass, black glass, optics, iron base, steel Measures the angle between crystal faces to determine the mineral makeup 15” high, horizontal circle is 7” diameter 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 Babinet goniometer featured here was designed for the measurement of interfacial crystal angles by means of a reflected collimated signal light. This instrument can also be used as a refractometer to determine the indices of refraction. This instrument sits on a 3-legged base. At the outer end of each leg there is a large brass thumb screw used to level the instrument. Rising vertically from the center of the base there is a 1” diameter, 9” high, central brass pillar. At the upper pillar end there is a slip joint that can be tightened and loosened with a knurled collar. When loosened the pillar can be extended from a height of 9” to a maximum height of 16” which increases the overall instrument height from 15 inches to 22 inches. There are four major components mounted at the top of the pillar. These include the collimating telescope, viewing telescope, 7” horizontal circle and the black glass adjustable crystal observation pad. These major components are stacked on and manually revolve around a 1-1/2” vertical precision shaft which is attached to the top of the 9” brass central pillar. The collimator bracket is at the bottom of the stack. Next up is the fixed (no rotation) 7” horizontal circle. The viewing telescope bracket is next and at the top is the black glass crystal observation pad. All the bearing clearances are almost perfect which is difficult to accomplish when stacking these items on top of each other. There is no vertical or horizontal play. All three components rotate smoothly for the full 360 degrees and are not too tight or too loose. The purpose of the collimator is to convert diffused light from an outside source to non-dispersing parallel light rays. The result is that the 6” long collimator focuses a rectangular slit-light beam with crisp clear edges on the crystal. The width of the light beam is controlled by an adjustable slit at the eyepiece end. When the eyepiece slit is almost closed the light beam appears in the viewing telescope as a rectangular white light about 1/2” high x 1/8” wide. There is also a spring loaded adjustment feature on top of the collimator eyepiece that will rotate the slit a few degrees from vertical if needed. The eye piece manually slides in and out 1/2” to adjust the focus. The collimator is held in a 2-1/2” riser attached to the end of it’s mounting bracket. Because the collimator bracket sits below the circle, the collimator is elevated by the riser so that it is directed to a point about 1” above the crystal mounting pad. The riser also has a tilting feature that allows the crystal end of the collimator to be moved vertically up or down for accurate projection of the light. A spring-loaded thumb screw is used to adjust the tilt. The collimator manually rotates the full 360 degrees around the horizontal circle and can be clamped to a groove that resides in the underside of the circle. The horizontal circle used to measure the crystal angle is 7” in diameter. It is silvered and engraved every 1/2 degree for the full 360 degrees and numerically engraved every 10 degrees. It is fixed to the central shaft and does not rotate. The circle is rigidly constructed with six thick spokes. The 6” long viewing telescope reads the slit-light signal after it travels through the collimator to the crystal face and is reflected to the telescope. The viewing telescope is focused at the eyepiece end with a thumb screw driving a rack and pinion motion. The reticle has a vertical and horizontal fine cross hair used when sighting through the telescope. Like the collimator, the viewing telescope is mounted on a riser so it is directed at a point about 1” above the crystal mounting pad. There is a one-minute vernier attached to the base of the telescope to read the circle. The vernier includes a clamp and tangent motion for fine adjustments. The crystal observation platform consists of a machined and polished flat black glass 1-3/4” diameter pad. The pad sits on a stiff spring. There are three precision leveling screws around the pad’s outer edge that are used to adjust the crystal so that it is perpendicular to the horizontal circle. The leveling screws also act against the spring to remove any play in the pad when adjusted. Mounted under and directly attached to the pad is a brass one minute vernier that reads the circle and manually rotates the pad as need to study the crystal faces. The vernier is equipped with a fine clamp and tangent motion. Before using the instrument the crystal faces may need to be polished to facilitate light reflection. The crystal is then centered and attached with wax on the black glass observation pad. The rotational axis of the crystal must be perpendicular to the pad. There is a precision pad tilt mechanism previously described to assist with this. In order to see the reflected signal it helps to darken the room. When the crystal is in position, the observation pad should be clamped and a reading of the horizontal circle noted using the vernier attached to the pad. Then the external light source, collimator, and viewing telescope are moved into a position where the rectangular slit-light or “signal” as it is called, is reflected off the crystal face and centered on the crosshairs of the viewing telescope. At this point, the telescope and collimator are clamped in place and the horizontal circle is again read by the observation pad vernier and noted as it may have been necessary to make some fine pad adjustments as part of the process. Next the observation pad is rotated to the next crystal face until the signal is visible and centered on the crosshairs of the viewing telescope. The observation pad is then clamped and the horizontal circle is again read by the observation pad vernier and the difference between the two face measurements equals the face angle. This instrument’s finish had been partially restored when it was acquired. The restoration work is excellent and we are not sure what is original and what was restored. The glass crystal pad vernier arm is engraved Ed Lutz Paris. Circa 4th quarter 19th century. Edouard Lutz manufactured many types of optical instruments for scientific observations and testing. He was active in the 3rd and 4th quarter of the 19th century in Paris France. The Smithsonian scientific instrument trade catalog collection has a Lutz 39 page catalog dated 1872 titled “Catalogue Des Instruments D’optique” written in French. Lutz wrote an interesting introduction which was part advertisement and is translated as follows. “In order to make myself useful to people who are willing to entrust their orders to me, I think I should publish a catalog of the optical instruments that I build. My house is already honorably known to MM teachers and scholars; she obtained a 1st class medal at the 1867 exhibition. I have been attached for many years to the many workshops in France and abroad; the best special interest builders and the most eminent scientist honor me with their confidence; I was therefore able to keep myself constantly at the level of scientific progress; I think I am able to satisfy the people who would be good enough to take charge of their orders with good construction and moderate prices. The teaching of optics in high schools and colleges requires a whole series of conventional devices. I have endeavored to establish a collection of models in good conditions of price, convenience and solidity. It often happens that a single well combined instrument may suffice for the development of a whole series of researches. Despite this, in most cases, we have built as many separate devices as phenomena to study. I think there is a lot to do in this direction, and I have tried to create simple and convenient models allowing us to repeat a whole series of experiences. I also build all instruments of physical optics and mineralogical optics, and put myself at the disposal of the scientists who will be kind enough to employ me in their research, as well for the mechanical part as for the work of the glass and the size of the crystals. You can finally find at my place a complete collection of microscopes, spectroscopes, telescopes, glasses of all kinds, and all sizes up to the largest, as well as various opthalmoscopes”. Jacques Babinet; Born 3/5/1794, Died 10/21/1872. He was a French physicist, mathematician, and astronomer. He was a brilliant scientist who made several important discoveries some of which were in the field of the polarization of light and the study of crystalline structures. One of his inventions was the Babinet optical goniometer which measured refractive indices.