The Ploessl ocular (“eyepiece”) is easily the most popular design in the hobby of astronomy. Regardless of focal length, from 40 mm to 4 mm, they are common because they are inexpensive and they reasonably support the limits of the largest amateur instruments. Better designs are available, but at a greater cost. A modest instrument under city skies usually will not benefit from the improved optics of an orthoscopic ocular. Thus, the Ploessl is the first choice, whether for a refractor, reflector, or catadioptric telescope. For all of its ubiquity, its inventor, Viennese optician Simon Georg Ploessl is not widely known among astronomers.
Say it Right
|Lithograph by Kriehuber|
The double-dots are a medieval shorthand for a little letter e that was placed over the o to show the shifted sound. Thus, the questionably undead cat is not as if in English long-o “Shro-din-jer’s” but umlaut-ö as if in English like “Shray-ding-er’s.” Ploessl also used the ligature œ (oe), a less common flourish.
If your typewriter has no umlaut vowels (ä ö ü) or a sharp-s (ß), you can use an e and a double-s. Thus, Plößl (which is how he spelled it) is accepted as Plössl or Ploessl, but spelling the name Plossl or saying it that way is wrong.
From the Microscopic to the Macroscopic
Georg Simon Plößl (1794-1868) was born in Wieden which had been an independent villa in the Middle Ages but by the 18th century already lay within Vienna’s shadow. He was the son of a cabinetmaker. And therefore an apprentice in his father’s shop, beginning as a lathe operator (turner). When he was 18, he left for the optical firm Voigtländer, starting on May 9, 1812 .
In 1823, he moved back to his father’s home and began his own laboratory and workshop for investigations into the production of optical instruments. In 1828 he was open for sales . At first, Ploessl made microscopes, and he soon became famous for them.
His company took off (and took a new direction) when he sold a microscope to Joseph Franz von Jacquin, professor of botany and chemistry at the University of Vienna. Von Jaquin introduced Ploessl to the astronomer Joseph Johann von Littrow for whom he built a telescope in 1830.
|Three-inch "dialytic" Refractor by Simon Georg Plößl|
At Viennese industrial fairs in 1835, 1839, and 1845, Ploessl was granted gold medals in recognition of the exemplary products of his workshop. In 1847 another gold medal was bestowed by the Emperor Ferdinand. The emperor also commissioned him to build a telescope which was gifted to the Vizier of the Ottoman Empire. (Some sources say that it was for the Sultan.) 
By 1850, in addition to the microscopes which were the primary production of his firm Ploessl had delivered refractors to observatories in Romania (Iasi; 6.4 inches), Hungary (Biczke; 8.5 inches), Greece (Athens; 8 inches), and Russia (Pulkovo; 6.4 inches). In 1851 the firm delivered an 11-inch f/11.8 refractor to the Vizier. Delivering up to 610x magnification, it was said to divide gamma Coronae Borealis: 0.6 seconds of arc at 4th and 7thmagnitudes. (The above is from 19th century reports. A modern paper identifies the Athens instrument as 162-mm (6.37 inches) .) Other Ploessl instruments have been identified. He also built dipleidoscopes for determining the moment of high noon, 12 o’clock post meridian.
All of his telescopes had objectives of crown glass. They followed the Fraunhofer design of a concave and convex lens pair, but did not use flint glass which was rarer in large, high-quality blanks. Consequently, the essential element in the design was the secondary lens system, a flint glass ocular that minimized chromatic aberration. That was Ploessl’s stellar achievement.
The Ploessl Ocular Lens System
Two pairs of lenses, match convex and concave curves, and the pairs are separated by a gap. The concave lenses are at the extremes, the convex face each other in the center. This design was not a happenstance. The Voigtländer firm was founded in 1763 when Johann Christoph Voigtländer received a monopoly charter (“protection decree”) from the Austrian monarchy to produce mathematical instruments. Fifty years later, Ploessl studied mathematics and optical theory while rising from apprentice to journeyman.
|Star Ware: The Amateur Astronomer's Guide|
to Choosing, Buying, and Using Telescopes
and Accessories, 4th ed.,
by Philip S. Harrington, John Wiley and Sons, 2007.
Three factors kept this system from becoming widely accepted. First, telescopes are durable goods. Better ones have been built since 1610, but the old ones still work. The Ploessl refractor in Athens served the university until 1940. Second, astronomy was a private pursuit for intellectuals of independent means. While Britain had its Royal Astronomer, few such public posts existed elsewhere. Ploessl built telescopes for state enterprises in Greece and Russia, but he built more for wealthy patrons in Hungary, Romania, and Italy. Only with the explosion of science in the 20th century was there any broad consumer demand for the instruments of empirical discovery. Third, as prosperous as the Ploessl firm was, it was a sole proprietorship. When Simon Ploessl was killed by a falling sheet of glass in 1868, there was no one to step into leadership. The firm continued until 1905, but there was no visionary to drive the effort.[*]
Enter Al Nagler
Al Nagler founded TeleVue. He gave an extensive interview to Astronomy magazine, which posted it as a blog and it is archived at http://cs.astronomy.com/asy/b/astronomy/archive/2015/10/13/the-evolution-of-eyepiece-developments-at-tele-vue.aspx The story is enjoyable and edifying. Briefly, he said:
While the Nagler was the first eyepiece I designed for Tele Vue, I feared entering the astronomy market as an unknown “kitchen-table” company with such an expensive eyepiece. This caution led me to produce a Plössl eyepiece first, to gain reputation, experience, and capital.
References and Further Reading
 The Hellenic Archives of Scientific Instruments at http://www.hasi.gr/makers/ploessl-georg-simon
 Looking at the Skies for 175 Years: The 162-mm Ploessl Refractor and the 400-mm Gautier Refractor of National Observatory of Athens, Panagiotis, Lazos and Tsimpidas, Dimitrios; XXXVII Scientific Instrument Symposium, 3-7 September 2018, Leiden and Haarlem.
 “Plössl-Mikroskope - ein Vergleich mit modernen Geräten,” by E. Steiner and P. Schulz, ©Naturhistorisches Museum Wien, download from www.biologiezentrum.at
 “The Achromatic Telescope, Dialytes, and Fluid Lenses--Nebula--Double Stars—Occultations” by the Rev. T. W. Webb, A.M., F.R. A. S., The Intellectual Observer, Groombrdige and Sons, London, No. XLIX, February 1866,
 “The evolution of eyepiece developments at Tele Vue,” Posted by Michael Bakich on Tuesday, October 13, 2015, A guest blog by Al Nagler. Astronomy.
[*] Addendum 14 December 2020. I found evidence of this. To make a short story long, I borrowed a Meade 10-inch Ritchey-Chretien catadioptric telescope from the local club. Researching it, I discovered a legal kerfluffle among Meade and some others. A discussion on Cloudy Nights took me to the website of retired telescope maker R. F. Royce (http://www.rfroyce.com/). At the top of one of his pages is this quote: "If the pure and elevated pleasure to be derived from the possession and use of a good telescope of three, four, five, or six inches aperture were generally known, I am certain that no instrument of science would be more commonly found in the homes of intelligent people." - Garrett P. Serviss, Pleasures of the Telescope, 1901. So, I followed that track. In his time, Serviss was famous as a popularizer of science, especially astronomy. When he was a night editor at The Sun of New York, one of his lecture tours was subsidized by Andrew Carnegie. His passing was noted in a long obituary in Popular Astronomy (August-September 1929). The Pleasures of the Telescope was first published by Putnam in 1901, and then went through several re-publications. Google Books and Hathi Trust provide the 1915 printing by D. Appleton and Company. Serviss described two kinds of eyepieces: positive (Ramsden) and negative (Huygens). He does not mention the Ploessl.
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