Alexis-Marie Rochon, Jean-Baptiste Grateloup, and the earliest cemented lens. Peter Abrahams. Presented to the Antique Telescope Society, Oct. 1999. The first use of lens cement in a multi element lens was a significant advance in the long development of small telescope optics. Cementing provides the benefits of mechanical stability, and especially of increased light throughput by the elimination of two glass-air surfaces from the four surfaces in a doublet, each of which can subtract 5 percent from the light beam. The provenance of this improvement was the subject of a recent discussion, and follow-up revealed a surprising obscurity to the innovation. As it happens, the issue was covered in a 1991 article by the versatile Allan Mills in 'Annals of Science'. Mills is with the department of geology at University of Leicester, and has published articles on Newton's telescope, sundials, and the camera obscura. In this long article on the history and use of Canada balsam, Mills discusses the work of Alexis Marie Rochon of France in the late 18th century. However, that is jumping ahead in the story, and the present research in fact went backwards in time from the present. Cemented lenses have been ubiquitous for many decades, and tracing backwards in time through the references shows almost all small lenses were cemented in this century and in the late 19th century. Carl Kellner developed his well known ocular, which used a cemented doublet, and announced the news in 1849 in the paper "Das orthoskopische Ocular". This would have been shortly after he left Repsold & Sons to work with Moritz Hensoldt, and the opening of their workshop was also announced in that paper. Kellner's ocular was an important advance, giving a wider field of view and better correction, orthoscopic referring to constant magnification over the field, or relative freedom from distortion. The Kellner eyepiece uses for the eye lens an over corrected cemented doublet with the flint facing the eye, and various forms for the singlet field lens. With up to 12 employees, Kellner built over 130 microscopes, many small telescopes, and 5 large telescopes, and was one of the founders of the optical industry in Wetzlar. He died at the age of 29 in 1855, and his shop was bought by Ernst Leitz. Joseph Max Petzval was a professor of mathematics at the University of Vienna when in the late 1830s he designed his first lens, aiming for a flat field and an aperture wide enough for the fast shutter speeds needed for portraiture, and motivated by the monetary prize offered for such a lens in a competition. The lengthy calculations involved in lens design motivated Petzval to enlist the help of artillery gunners, which in those days was one of the few professions to practice computing. It took Petzval, a corporal, and eight gunners, six months to complete the job, producing two variations on the Petzval lens. This f 3.6 lens had at front a conventional telescope objective, an f5 cemented doublet; and at the rear an air spaced doublet, for the separated elements were needed to control spherical aberration and coma. Johann Friedrich Voightlaender made the first such lens for Petzval in 1840, a 150 mm f 3.6, delivered mounted in a camera, a major improvement which was about 20 times faster than the lens by Charles Chevalier commonly used at the time. The Chevalier used two cemented doublets but was an inferior lens that is rare today; and the Petzval lens is the first widely produced cemented lens. It was made until the 1920s, but many examples are not labeled as such to avoid patent restrictions. In the years around 1815, Joseph Fraunhofer developed his famous telescope objective, which was air spaced and used different radii for the two inner surfaces to achieve best correction. Over the next twenty years, various opticians learned to make doublets with matching curves on the inner faces while maintaining an adequate degree of correction, and the matching profiles allowed them to cement the lenses. However, the details of these lenses and lens makers remain to be uncovered, and for telescopic use the finest lenses larger than an inch or two were air spaced. The time before Fraunhofer was the era of the introduction of the cemented lens, using for a bonding agent an import from the New World, Canada balsam. There were many balsams available at apothecaries, but Canada balsam was especially useful in optical instruments because it has an index of refraction that is higher than most resins, up to 1.55 in commercial products, within the range of crown and light flint glasses. This particular range of refractive index also played a part in the development of the petrographic microscope, for it lies between quartz and orthoclase, and the use of balsam to mount thin sections of rocks allows the differentiation of these two common minerals. It also allowed the construction of the Nicol prism, one of the first polarizing elements, because the index of balsam lies between the index in calcite of the light polarized in one direction and the light polarized oppositely, and thus a layer of balsam can separate the two polarized beams. For two centuries, Canada balsam was the predominant material used to cement optical glass. Canada balsam is derived from the bark of the tree Abies balsamea, widely found in North America. In mid summer, the bark is pierced with a sharp tube where blisters of balsam have formed, and an active collector can accumulate a half gallon in a day. When stored, it solidifies into a resin, soluble in turpentine and many other solvents; and steam distillation results in a harder resin. Balsam was exported in large quantities from the New World, because the tree cannot thrive in Europe. Among those who utilized this useful new product was Alexis Marie Rochon, a footnote in the English language documentation, but a historical figure who greatly rewarded a little investigative work. Alexis Marie Rochon (1741-1817) was an experimental, practical physicist; and also an instrumentalist and tinkerer. He held the title of Abbe until the Revolution, and wrote a series of memoirs on optical instruments, which were given to the Academie in 1766. In 1771, he became a member of this French Academy of Sciences. King Louis XV had assembled a collection of instruments at the Chateau of La Muette at Passy, and Rochon was placed in charge of the optical instruments in 1775. He was also astronomer-optician to the French Navy, and director of the observatory at Brest. His interest in navigation resulted in his most famous work, the first book by a European on a voyage to Madagascar, with much anthropological content, and he returned from Madagascar with quartz crystals that he made into objectives. He experimented extensively with lenses and also was interested in lighthouses. Rochon studied glassworking in order to minimize defects in glass, devised methods of casting flint in clay molds, using iron tongs; and with the Saint-Gobain glass plant, made crucibles lined with platinum. He wrote a brief history of achromatic lens, which appears in Recueil de memoires sur la mecanique et la physique, 1783. The instrumentalist Rochon was very prolific. He improved the heliometer of Bouguer, devised an engraving machine, and improved Hadley's reflecting quadrant by using achromatic prisms instead of mirrors. Rochon's most important instrument was the prismatic micrometer, an attachment to a telescope with achromatic prisms that used the double refraction of quartz to produce two images that were made to coincide by moving one optical element along a scale. One version had one fixed prism and one rotatable prism mounted on the telescope objective that was turned to bring two stellar images into contact. The use of two prisms resulted in excess light loss, and the next model used one prism that was moved laterally across the optical axis to bring the two images together. A final model used two different quartz prisms, one gave a maximum and one a minimum of double refraction, and this model was introduced at the Academie in 1771 and came into general use. The idea was also developed into a rangefinder. Rochon directed French efforts to improve reflectors and achromatise refractors. (Due to the nature of translated references, it is often unclear whether Rochon supervised or fabricated a project.) He worked with Ferret to make better flint glass, and made a triplet of 7 foot focal length and 6 inch aperture, but only the central 4 inches was usable. He realized that the distortions were a result of incorrect curvatures, and devised the diasporametere, which measured the index of refraction of glass using a variable angle prism. He is mentioned as the first to use an objective prism to observe the spectra of a field of stars, circa 1776, and wrote on stellar spectra in contradiction to Newtonian ideas. He did not detect some details of the spectrum and was mistaken in much of his spectral work. A telescope of his manufacture was used during the 1769 transit of Venus in California. Under Rochon's direction, Noel Simon Carochez, at Passy, made an 8 inch platinum mirror, used in a Gregorian of 6 foot focus at up to 500 power, with an image reported to be of high quality, and presented to the Academy of Science in 1785. Platinum telescope mirrors were used in a few other instruments by a French opticians, plans were made for a much larger instrument that was never completed, and Rochon wrote on these mirrors in one of his published memoirs. Rochon became interested in the fluid optical elements of the eye and experimented with combinations of glass and fluid elements for achromatic telescope objectives. With a committee including the Cassini brothers, Rochon took an achromat doublet with elements spaced by one half inch and inserted a plate of unworked glass. The group attempted to read a printed page at various distances, and found that by filling the space between elements and plate glass with water or oil, they could read the print at a greater distance. Rochon deduced that a fluid filled lens can correct defects of lens surfaces. This led to his use of liquid films between matched glass elements, using water and different oils, and the consequent problems of seepage, evaporation, and oxidation. Rochon was assisted in this work by Jean Baptiste Grateloup, an artisan and not an optician, who was born in Dax in 1735 and died 1817 in the same town, where he was conservator in the department of mineralogy. Grateloup made a lifelong study of physics, and was noted for his paintings on enamel, and for many ingenious inventions, especially an engraver with which he made many portraits noted for the fine technique they revealed. It was Grateloup who is said to have suggested the use of substances other than liquids between glass elements, and who began experimenting with resins that were solid at room temperature but could be melted in application. The first successes were made with mastic en larmes (teardrop resin), a common pine resin adhesive made by evaporating the turpentine, filtering, and pouring the melt into cold water to make drops of a resin that is very similar to modern Canada balsam. He worked with the trained optician Etienne-Antoine Putois (1763- ) in 1785 to develop the technique. They heated and cooled the resin to various densities, to match the index of refraction of the crown glass. Putois melted mastic with fire onto a lens surface and laid the matching surface onto the liquid. As an experiment, they left a lens imperfectly polished and cemented only half of the surface, finding that transmission was greatly improved in the cemented portion, and claiming that the inner surfaces of a cemented doublet needed no polishing. They had to solve problems with mastics, including the transparency, color, melting properties, and cooling to solid without crystallizing. Rochon noted the reduction in reflected light and ghost images achieved in these lenses, and proposed objective lenses of 5 elements and 4 cemented joints. Cemented achromats of 61 mm and 90 mm aperture were made by Grateloup & Putois in 1786. Their results were reported by Grateloup to the Academy of Sciences in 1787 and published as a pamphlet, Memoire sur l'optique, in 1788. Both were granted large monetary prizes and awards, in recognition that cementing could be used to correct surface irregularities and to reduce polishing to that of the two exterior surfaces. The technique was quickly adopted by other French opticians, including Carochez, Lerebours, and Rochette. Putois went on to introduce to France the British developments in fluid element lenses. It remained for David Brewster to measure the refractive indices and dispersive properties of various balsams and oils. Brewster also made microscopes, both simple and compound, with lenses of drops of Canada balsam. For a simple microscope, he pierced sheet metal plates with tiny holes, placing droplets of molten balsam in the hole. To make a multiple lens microscope, he needed plano convex lenses. Brewster dropped balsam onto thin polished glass plates, smoked the lower surface, removed the soot beneath the droplet so light could pass only through the lens, and stacked four or more of these plates. Many or most of these earliest cemented lenses deteriorated over a few years, and indeed most modern balsamed lenses craze, bloom, decement, or otherwise deteriorate over a longer interval. However, balsam was by far the predominant optical cement until very recent times. Cementing became critical later in the 19th century, when complex visual instruments such as prism monoculars and binoculars were built, where a series of uncoated optical surfaces could reduce light throughput by half. By reducing exposed surfaces, cementing reduces haze and ghost images from reflections, and allows the use of thinner flint elements. It would never be appropriate for all lenses, since it limits the lens designer or fabricator to matching curvatures on the two inner surfaces, and can not be used on lenses larger than perhaps six inches. When balsam cement dries, it slightly shrinks, which can distort delicate optics or cause Newton's rings. During WWII, aerial photography lenses were flown at high altitudes, in temperatures below -20 degrees Celsius, where they frequently separated or 'bloomed' in a snowflake pattern. UV exposure could age balsam or cause fluorescence, and this combination of problems led to the development of synthetic resins, which have largely replaced balsam as a lens cement. The easy reversibility of balsam joints has guaranteed its survival in experimental lenses, student projects, and other work where easy decementing is important. (Biographie Universelle.) Paris: L.G. Michaud, 1824. (vol. -, p337-47, Rochon. vol. -, p38-9, Grateloup) Blay, Michel. Note sur l' "Essai sur les degres de chaleur des rayons colores" de l'abbe Rochon. Revue d'histoire des sciences 38 (1985): (37- 42) Brewster, D. Treatise on New Philosophical Instruments. London, 1813 (Catalogue General des Livres Imprimes de la Bibliotheque Nationale.) Paris: Imprimerie Nationale, 1938. (vol. 43, p1068-72) Chapin, Seymour. "In a Mirror Brightly": French Attempts to Build Reflecting Telescopes Using Platinum. Journal for the History of Astronomy iii(1972), 87-104. fn 12 Daumas, Maurice. Scientific Instruments of the Seventeenth and Eighteenth Centuries and their Makers. Mary Holbrook, editor and translator. London: Portman, 1989. Fauque, Danielle. Alexis-Marie Rochon (1741-1817), savant astronome et opticien. Revue d'histoire des sciences 38 (1985): 3-36 French, James. The Balsam Problem. Transactions of the Optical Society, 19 (1918), no. 5, 143-63. Gillispie, Charles. Science & Polity in France at the End of the Old Regime. Princeton: Princeton U. Press, 1980. (p6, 85, 321 Grateloup, Jean-Baptiste, Memoir sur l'Optique. Moyen de perfectionner les objectifs des lunettes achromatiques. Paris, 1788. Mills, Allan. Canada Balsam. Annals of Science, 48 (1991), 173-185. Poggendorff, J.C. Biographisch-Literarisches Handwoerterbuch zur Geschichte der Exacten Wissenschaften. Leipzig: J. Barth, 1863. (vol. 2, 670-1) Rochon, Alexis Marie. Recueil de memoires sur la mecanique et la physique, 1783 ======================================================= Further Notes: ====== Robert E. Cox, cementing lenses, Telescope Making #28, p16-19 see Daumas & Turner: Lenoir see Daumas 286, 338 Grat; 37,100,128,153,158,168,258,268,271 Roch ================================ Alexis Marie Rochon. 1741-1817. Recueil de memoires sur la mecanique et la physique, 1783 Jean-Baptiste Grateloup. 1735-1817. Memoir sur l'Optique. Moyen de perfectionner les objectifs des lunettes achromatiques. Paris, 1788. Etienne-Antoine Putois (b. 1763) Noel Simon Carochez ============================ In the 1976-77 Annual Report of the Royal Observatory, Edinburgh, page 21, report on the UK Schmidt telescope in Australia, installation of a new achromatic cemented doublet Schmidt corrector plate, 1.2 meters in diameter, with the cemented surfaces and one outer surface being aspheric. It was figured & assembled by Grubb Parsons, and provided a major improvement in the red, infra-red, and ultraviolet, allowing use of light from 330nm to 1000nm. In a classic of understatement, it "is believed to be by far the largest cemented doublet lens ever made". Also in that year, the first successful color photos were taken with this Schmidt, 'greatly assisted' by David Malin. Halton Arp was another user. ================================ Grateloup( Jean Baptiste) (Biographie Universelle.) Paris: L.G. Michaud, 1824. (vol. -, p38-9, Grateloup) Born at Dax in 1735 and died February 18, 1817 in the same town, where he was conservator in the department of mineralogy and was also a member of several scientific societies. All his life he studied physics and made a name for himself by ingenious inventions of which the main one is an invention which engraves beautifully and has not yet been equaled . The daintiness, the gracefulness, the purity of the design together with the charm of the skill of coordinating shadow and light and at an extreme finish characterise his engravings, which represent the following portraits: 1. Jean Baptiste Bossuet, full size and bust after Rigaud. 2. Fenelon after Vivien, 3. Jean-Babptiste Rousseau after Aved, 4. Jean Dryden after Kneller, 5. The Cardinal of Polignac after Rigaud. 6. Miss Lecouvreur in the role of Cornelie, after Drevet. 7. Descartes, after Hals. 8. Montesquieu, after Dassier. These engravings are recognized as masterpieces (See : dictionary of ancient and modern engravings, by Basan, Volume I , page 250) 1809, the conservator of engravings and woodcuts said, when thanking Grateloup for the gift of the portrait of the cardinal of Polignac the following:' You have always been alone in your speciality. Nobody has dared to imitate you and I believe that's just as well. Your beautiful collection has a distinguished place among the masterworks which have been entrusted to me.' Grateloup also excelled in paintings on enamel and his works in this medium have become very rare. Another discovery which doesn't give him any less fame is that of the improvement of achromatic objectives, which was invented by the famous English optician Dollond. This discovery, developed in a commemoration which the author read on December 5, 1787 at the Academy of Sciences of Paris was approved by this Society and the commemoration was judged to be worthy to be printed in the collection of foreign scientists; and in 1793 upon the recommendation of its members the same academy ' considering the advantages which resulted for the optics by the gluing of achromatic objectives with the tear glue(?) to correct the surface irregularities as well as to reduce the work of the achromatic objectives to that of the two exterior surfaces, was of the opinion that, according to the law of September 12, 1791 Grateloup deserved the maximum national reward and honorable mentioning, which was adopted.' ===================================================== 6