It is so old and small and fragile, this image on paper the size of a postcard, that its existence seems something of a miracle.

The first known view of Pittsburgh, a watercolor painted in 1790, is one of the first we encounter in "A Panorama of Pittsburgh," the splendid, landmark exhibit of 19th-century prints at the Frick Art Museum in Point Breeze. Organized in honor of the city's 250th anniversary, it is the first scholarly exhibit to explore the early printed views of Pittsburgh, documenting 135 works drawn from 29 sources here and around the country -- libraries, museums, universities, corporations, print shops and a handful of passionate private collectors.

Lewis Brantz's "Pittsburgh in 1790," a delicate pen-and-ink wash and watercolor of the Point, was drawn from the vantage point of what is now the South Side, showing the barracks of Fort Pitt and how the village of Pittsburgh grew first along the Monongahela shore. A native of Germany then living in Baltimore, Brantz was investigating the commercial possibilities of the Ohio and Mississippi rivers when he passed through Pittsburgh, noting on a visit in 1785, "The view enjoyed at this place, from two elevated spots is, in truth, the most beautiful I ever beheld."

What would he have thought of its evolution, so clearly documented in this exhibit, from pastoral confluence to dense metropolis with black smoke wafting into the sky? In 1853, the Brantz watercolor was made into a print, also in the exhibit -- as was the only other known 18th-century view of Pittsburgh, a small (about 5-by-7-inch) pen-and-ink wash. It was created by Joseph Warin, a cartographer who accompanied French Gen. Georges Collot on a covert mission in 1796 to survey the political, commercial and military strength of the western rivers in the event of war with the British over the French-controlled Louisiana Territory. Now in the collection of The New York Public Library, Warin's "Vue de Pittsburgh" was made into an engraving that appeared in 1826 in Collot's published journal -- as did one of the earliest known maps of the town.

William Schuchman published his tinted lithograph of Pittsburgh in 1859, 10 years after he became the first lithographer to open a print shop in the city, at Third and Market streets.

Here and elsewhere in the exhibit, it's thrilling to see in the original what we have seen heretofore only in small reproductions in books and magazines, if we have seen them at all. The details are dazzling, in their own quiet way. And it's fun to learn the back stories, both in the generous object labels accompanying each print and drawing and, more expansively, in the fully illustrated, color catalog written by guest-curator Christopher W. Lane. Co-owner of the venerable Philadelphia Print Shop, one of the lenders to the exhibition, Lane traces the evolution of printmaking in Pittsburgh and includes a long list of 19th-century printmakers here -- designers, draftsmen, lithographers, and wood, copper and steel engravers.

The catalog ($24.95) also provides the first comprehensive list of 19th-century printed views of Pittsburgh -- 423 of them, more than three times the number in the exhibit, making it a valuable research tool long into the future.

Staged in three galleries, the exhibition is presented in five sections that for the most part flow together seamlessly: Pittsburgh Before the Great Fire of 1845, Views from Books and Magazines (displayed in their original contexts in vitrines), Documentary Prints (memorable current events), Illustrated Newspapers (disasters and labor strife) and Frameable Views (panoramic and other prints).

This 1891 chromolithograph of St. Stanislaus Kostka Church in the Strip District shows the original bonnets atop its towers. The bonnets were removed after a 1936 explosion across the street weakened the towers.

The first cityscape print of Pittsburgh published at the time of its drawing was a picturesque engraving (complete with South Side cow) by Philadelphia artist George Lehman. It appeared in 1829 as the frontispiece of Samuel Cumings' "The Western Pilot," a guide to the Ohio and Mississippi rivers. There were numerous factories active in Pittsburgh then, but Lehman gives no hint of the city's industry, Lane notes. Lehman returned a decade later and sketched the town from Grant's Hill for a state-sponsored geologic survey -- oddly one of the few renderings made from that vantage point, considering it was the city's favorite picnic ground and promenade. Lane suspects the image's accuracy owes to Lehman's use of a camera lucida -- a mirrored optical device that appears to reflect an image onto paper. Lehman's is the best view of Pittsburgh before the 1845 fire, which wiped out a third of the town -- the oldest part, along the Mon.

Other prints give a sense of the role Grant's Hill played in the city as a sort of Acropolis with its own Greek temple -- the county courthouse of 1841. One of them is Edouard Willmann's 1866 steel engraving, which shows the hill's gradual rise beginning at Smithfield Street. There were few buildings in the block in front of the courthouse, protecting the view. How different from today, about a century after "the Hump," the last vestige of Grant's Hill, was shaved away and the 20-story Frick Building went up in front of the present courthouse.

The Greek Revival influence, once so strong here and of which so little remains, also can be seen in the small sketches of outlying estates that embellish the hand-colored wall map of the city in 1852, one of the first images the visitor encounters and, at 42 by 73 inches, one of the largest. William Eichbaum's plantation-style home, with two-story columns, then sat majestically on the Oakland hillside where UPMC Montefiore is today.

Seven years later, James Palmatary produced his extraordinary, sweeping lithograph of the city in four sheets that form a large rectangle, 43 by 85 inches. As with most panoramas of Pittsburgh, the vantage point is from Mount Washington, yet details on buildings as far away as the Bluff, the Strip and the North Side are evident. How did he do it, we marvel. He sketched the buildings at street level, Lane writes, then used "a standard formula to modify the images so to appear as though seen from above."

In 1874, Otto Krebs published this three-tone lithograph by J. Warnich from Krebs' original drawing. Krebs soon reprinted the image with the addition of the Point Bridge, completed in 1877 across the Monongahela.

Bruce Wolf, whose extensive collection of Western Pennsylvania prints is represented in the exhibit, discovered the unattributed Palmatary print on the wall of the coat room at the Duquesne Club several years ago, led the research of its history and championed its restoration.

Drawing in part on Rina C. Youngner's book, "Industry in Art -- Pittsburgh 1812 to 1920," "A Panorama of Pittsburgh" is the third publication and second exhibit (along with the Westmoreland Museum of American Art's "Born of Fire") to showcase Pittsburgh views in the past two years, creating a trilogy of independent but related work that adds immeasurably to our understanding of how the city was interpreted by artists as it grew.

Delight is in the details, so if you use reading glasses, bring them. And you might as well pack your magnifying glass, too. The museum thoughtfully provides plastic ones, but they are a tad small. You'll want to see and savor the minutiae: not just the expected architectural details but also, say, the pigeons flying around the courthouse cupola and the sausages and hams hanging in the market house, both on the Diamond we now call Market Square, in J.P. Robitzer's print from the 1870s recalling a scene of several decades past.

"Panorama," which continues through Oct. 5, is a reminder that artists who devote themselves to city views also are documentarians and historians whose work may be far more valued in the future than it is today.