A FEW WEEKS ago, I had come home from some evening performance of other, and seeking innocuous release, flicked on the TV set. I had started to undo my tie, as I recall, but what I saw there kept me from completing that task for another hour and a half.

It was the beginning of Sergei Eisenstein's epic film about medieval Russia, "Ivan the Terrible, Part I," which was broadcast on public television as a part of the Exxon-supported series of film classics inaugurated this year. I didn't know this righ away - I had never before seen the film, and had forgotten about its scheduling. But what was unmistakable from the first instant was the phenomenal power of the imagery.

PBS, in mounting the series, has gone to a great deal of trouble to secure the cleanest and most complet prints available, and has newly titled the foreign pictures, very legibly, especially for TV. The brilliance of the black-and-white tones of "Ivan" was startling enough in itself. It was Eisenstein's mastery of the medium, however, that really held me - faces, in close-up, of such charismatic intensity, each separate frame a pictorial composition of such striking dramatic authority that I stood mesmerized , rooted to the spot. In less than a minute of screen time, I knew that whatever this was, it belonged among the timeless riches of motion picture art.

"Ivan the Terrible" is one of those cases in which any small segment of the work, chosen arbitrarily or accidentally, would testify to the same grandeur of conception and technical command. This is partly a consequences of Eisentein's methodology. The film deals in narrative from with the history of Ivan's turbulent reign, but though it proceeds discursively, the experience of watching the film is more like strolling through a gallery of paintings than observing a continuous dramatic plot. The individual shots are so meticulously staged and so heavily stylized that perhaps the best description of the film is "opera without singers," as Pauline Kael once put it. The emotional input of Prokofiev's musical underpinning - one of the model film scores of all time - helps to reinforce this impression. The actors may not sing, but their photographic semblances offer visual melody and counter-point.

Once the thought strikes, it is easy to summon up numbers of related instances - films like Murnau's "Sunrise," Griffith's "Intolerance," Stroheim's "Greed," Gance's "Napoleon," and Renoirs's "A Day in the Country" are some that come quickly to mind, But the principle works as well with pictures beyond the pale of established classics. A movie like Sammuel Fuller's "Under-world U.S.A." for example, - a fast hard-knuckled crime melodrama - is obviously no "King Lear," but the tense, almost visceral impact of Fuller's punchy visual style gives it a distinction that not only raises it above the common run of its genre, but is also instantly identifiable from the opening scene.

One of the defining attributes of a great work of art is that it is a whole greater than the sum of its parts. The totality of Beethoven's Ninth Symphony, for instance, or "Citizen Kane" or Balanchine's "Jewels," is not just a sum of words or pictures or steps, but that and something more - an elusive, mysterious, perhaps indefinable something which irradiates every corner of the work, and which, for lack of a more concrete term, we call genius.

It is also frequently true, however, that artistic greatness can reveal itself in tha minutest sampling. A part, that is - sometimes even the merest fraction - of a truly great work can imply the whole, or at least, the quality of the whole. One doesn't need to hear all of Stravinsky's "Rite of Spring" to realize that one is listening to an extraordinary musical composition. A few bars will suffice, and it hardly matters where or how those bars are selected. This tells us something about the powers of human preception, of course, but it also says thing about the nature of genius.

If the whole of work is implicit in a part, it is because the artist has somehow been able to endow that part with the very traits - structural economy and cohesion, thematic consistency, emotional insight, and that twinge of the unexpected which always accompanies creative originality - which are the signature of achievement. We see and hear a fragment, and our imaginations make the leap to the grand design, extrapolating from detail to completed pattern.

Sometimes it is only the "privileged moments" of a work that have the power to evoke the majesty of the whole. I'm thinking of the final pages of Liszt's B Minor Sonata, for example, wherein the germinal motives of the entire opus return in a distilled, sepulchral gloom, only to disappear in a last incredible, transfiguring cadence. But sometime the hand of the master leaves an imprint so strong, so unyielding, that a small exverpt hints not only at the larger immediate context but the whole of a life's work. If all of Sebastian Bach's music had perished in the flames, say, but for the final chorus of "The Passion According to Saint Matthew," would we have any doubt as to the stature of its composer?

The mention of Bach brings to mind another, kindred phenomenon - sometimes the briefest, most "minor" work is sufficient to establish the transcendance of its author. There are, for instance, Bach keyboard pieces no more than a page long - a little Prelude in C Minor, for instance, often studied by children - which clearly could not have been written by anyone of lesser mettle. Similarly, if Schubert had left us nothing but the song, "Ungeduld," to pick one of any number of possible exhibits, or if Debussy had bequeathed nothing but "Reflets dans l'eau," these composers' claims to eminence would not be seriously diminished, however much we might lament the meager size of their legacy.

What is treasurable in the arts rarely admits of quantitative measure. A few beautifully woven threads can be as eloquent as a wall of tapestry.