One way to observe the difference between a very good pianist and a genius is to compare Glenn Gould's new recording of Sibelius' piano music with the recording made for Musical Heritage some time ago by David Rubinstein. But first, it might be a good idea to take a quick look at the background.

Jan Sibelius is best-known as the composer of "Finlandia," a granite monument of musical nationalism; of the brief, melancholy "Valse Triste"; seven symphonies (of which only Nos. 1, 2 and 5 are heard very often), a great violin concerto and a bit of prgram music dealing chiefly with Finnish legends. Most of his fans have probably never heard his piano music, although more than 10 percent of his published work (17 opus numbers in a total of 119) is written for that instrument. Those who have been aware of it at all have dismissed it as salon music, negligible in quality and hopelessly old-fashioned in style.

Rubinstein's recording was a revlation. It took the music seriously and presented it in an interpretation of considerable technical skill and anaytic depth; it made it clear that under the unpretentious surface of these short pieces there was music of real substance, music that demanded and justified serious attention.

In "Glenn Gould Plays Sibelius" (Columbia M 34555), the sense of revelation escalates sharply. To sum up the effect briefly: Rubinstein made these little pieces sound like good music; Gould makes them sound like Sibelius. He does this primarily by rethinking the music, calculating every nuance in the light of the composer's other work, indulging in extremes of tempo, dynamics and accent that push these seemingly slight works to their expressive limits.

The program includes the three Sonatinas of Opus 67 and "Kyllikki," Opus 41, which is labeled "Three Lyric Pieces" but sometimes sounds like a small tone-poem for piano. Kyllikki is the abducted bride of Lemminkainen, but the program notes (Gould writes his own and they are superbly literate) will tell you all you need to know about that.

In his piano interpretation, through carefully calculated gradations of pace, dynamics and even of the weight of individual tones within a chord, Gould produces effects that sometimes recall strikingly the spare, highly personalized textures of the composer's orchestral music. The effect is enhanced by a recording technique that is sure to be controversial but should not be dismissed as mere trickery. In a brief addendum to the liner notes, producer Andrew Kazdin calls it "acoustic orchestration" and explains that the performance was recorded "in a simultaneous variety of perspectives" and then edited to give each passage its ideal acoustic context, like a movie camera shifting from long shots to close-ups.

This will infuriate those who think a recording should try to duplicate exactly what you hear from a particular sent in a concert hall, but it seems a logical development in the recoding medium; the flexibility is there, and why not try to use it? It works particularly well with Gould's performance, which also examines the music "in a simultaneous variety of perspectives."