ONLY 15 years ago, two distinguished human dynamos, President Lyndon Johnson and millionaire art collector Joseph Hirshhorn, reached an agreement. It led to Mr. Hirshhorn's donation of his collection of over 6,000 pieces of sculpture and modern painting to the public. After a protracted congressional dogfight and years of complaint from the art world, the Joseph H. Hirshhorn Museum and Sculpture Garden opened finally on the Mall in 1974. Now, seven years later, "The Hirshhorn" has become recognized widely as a museum of stature.

Mr. Hirshhorn, who died yesterday at the age of 82, a Latvian-born immigrant who made his first fortune while still in his 20s, was a figure more easily described--and often caricatured--than understood. Once business success had provided sufficient funds for the task, he spent his adult life acquiring paintings and sculpture with a decisiveness and rapidity that became legendary in art circles. Despite the inevitable number of clinkers, a remarkably large percentage of the collection (much of which Mr. Hirshhorn assembled before obtaining any professional guidance) has weathered the skeptical scrutiny of critics.

Nor did his role as a bluff, exuberant aesthetic benefactor preclude in his personality an impressive element of restraint. Thus the same man who insisted on affixing his name to the museum that held works that he called "children" also chose for himself a spartan six-line summary in "Who's Who In American Art": "HIRSHHORN, JOSEPH H. COLLECTOR . . . American 19th and 20th century painting, European 20th century painting; sculpture from antiquity to the present."

Who, then, was Joseph Hirshhorn? Despite all the publicity he enjoyed over the past decade and a half, as well as a small number of books and articles on his career, much remains to be examined about the man and his passionate pursuit of art. Among the great American figures in the annals of acquisition, whose concern for art patronage evolved usually after at least a few generations of family residence in the New World, Mr. Hirshhorn was the exception--a first-generation Carnegie of art collection. Understandably, he expressed often his awareness of his roots as an impoverished immigrant. As a patron of art, Mr. Hirshhorn proved a genuine and valued eclectic, one who chose what seemed to his taste the best examples of those divergent styles that attracted him. If only because of the high quality of that eclecticism, The Hirshhorn will continue to draw appreciative millions to the museum that bears its sponsor's name--deservedly so--in perpetuity.