I still haven’t figured out whether the gradual shifts in tone within the nominally white ground of Bridget Riley’s Pause, 1964, are real or only apparent, chemical or optical. I am certain, though, that in her Cataract 3, 1967, the colors actually do change from top to bottom. It’s just that I can’t quite put my finger on where the shift takes place. And while I keep looking to find out—sometimes until my eyes ache, which doesn’t take long—I’m not sure I want to know. Their underlying illusionism is mental, not optical: the intimation that understanding their operations would explain their significance. Behind the eye-catching spatial and coloristic effects of Riley’s Op paintings from the ’6os and ’7os is always a system, and sooner or later you are tempted to investigate it—to try to transcend experience in favor of knowledge. Sometimes you can identify the mechanism at a glance; elsewhere, as in Pause and Cataract 3, you have to work at it. But in either case, what you know once you know the system has nothing to do with understanding the experience that aroused your curiosity in the first place.

Riley’s illusionism is very distinct from that of Wojciech Fangor, another painter featured in MoMA’s notorious “Responsive Eye” exhibition in 1965 . This Polish émigré may be obscure now, but in 1970 he was the subject of a major exhibition at the Guggenheim in New York. In the five works recently on view at Mitchell Algus—one from 1963 , the rest dated between 1969 and I972—Fangor reveals himself as a thoroughgoing sensualist, building up simple, solid forms (mostly circles or wave patterns) only to dissolve their boundaries into intangible halations as airy as cotton candy. Existing in some netherworld between Op and Color Field, these paintings arc what Ugo Rondinone was unknowingly quoting when we all thought he was quoting Kenneth Noland—but they are all the more compellingly disconcerting because what looks like spray painting has actually been painstakingly worked up in oil with soft brushes, so that its very disembodiedness feels unreasonably palpable. Looking at them is like falling into a beanbag chair: They are unresisting yet enveloping.

Riley’s paintings don’t offer that kind of comfort. Mostly they’re about feelings of confusion, instability, or loss (thus her penchant for titles like Disturbance, Arrest 2, Deny II). Those experiences are related to what early modern aesthetics called the sublime, which Roland Barthes later reconfigured as jouissance. And what may be more disturbing than the experience itself, which is pleasurable in a quasi-erotic way—a sort of swooning—is the certain knowledge that it was not germane to the work’s making, which (as we see confirmed in some working drawings shown at PaceWildenstein) must have been cool and systematic, though possibly furiously so. Does that make the artist’s stance with respect to the viewer one of generosity—or domination? Looking at Riley’s paintings, we can barely distinguish the two.

The art world dotes on young women, ignores them when they’re middle-aged, sometimes idolizes them when they’re old. Riley has experienced the first two situations; now it seems that she has lasted long enough to be lionized. But the question is, are we willing to have her whole, or do we just want the young Riley all over again? Last year the Serpentine Gallery in London showed her work of the ’6os and ’ 7os; now Dia has put its imprimatur on the same period, extending its historical reach just a bit, to 1 984 (and leaping forward to include a new black-and-white wall painting, Composition with Circles 2, 2000). In the ’70s, Riley reached new levels of subtlety and complexity in great paintings like Veld, 1971, with its strict and insistent diagonal lines emitting phantom colors, and Song of Orpheus 5, 1978, whose slinky, twisting bands conjure rising and falling volumes as present and ungraspable as waves in the ocean. Here what seemed shockingly raw and naked in her early black-and-white paintings—their palpable, almost bodily claim on the beholder—turns suave and becomes alluringly veiled yet somehow remains as potent.

PaceWildenstein’s selective update suggests that there are reasons why Riley’s later work remains overlooked. In the ’8os, just as she was being “postmodernized” in the early work of Philip Taaffe, Riley acceded to modernist orthodoxy by playing down perceptual illusionism and spatial activation in vertical stripe paintings (like Samarra, 1984, at Dia, or Blue Quiver, 1983, at PaceWildenstein) whose intricate color sequences and rhythms are virtuosic but lack the visceral grip of her previous work. Toward the end of the decade she began complicating these verticals by superimposing diagonals on them, leading to works like Dark Light, 1991, in which Cubist-like faceting becomes the medium for shimmering flows of color that ratify the artist’s claim that the ultimate source for her work was always Impressionism. In the ’90s the verticals were replaced with waves, in blandly Matissean works like Parade 1, 1999-2000. Here big areas of color, new to ’Riley, reveal a surprisingly dull sense of surface. Unable to indulge in the sensualism that was second nature to an artist like Fangor, she’s abandoned radical bliss for a pleasure that feels secondhand.

Barry Schwabsky is a frequent contributor to Artforum.