"Parallax" is a seemingly obscure trigonometric concept that astronomers use to measure distances to nearby stars, but it derives from an earthly optical phenomenon that is easily grasped. The operative principle is that the location of nearby objects will appear different against a distant background when the position of the observer changes. In Lestrygonians Bloom thinks of it as a "Greek" word that "I never exactly understood," comparable to "metempsychosis." In this hellenic book, both terms promise to hold multiple applications to ordinary life, just as the adventures of Odysseus do.
The calculation of parallax takes advantage of the dismaying fact that, for the objects studied by astronomers (and, in less obvious ways, for all objects of sight), location can never be more than relative or apparent. The universe has no center, no boundary, no fixed points of reference. Planets and stars and galaxies drift toward one another, and away from one another, but no universal grid assigns them to absolute locations. Ithaca captures the spirit of this vast cosmic rootlessness when it mentions "the parallax or parallactic drift of socalled fixed stars, in reality evermoving wanderers from immeasurably remote eons to infinitely remote futures."
The ingenious human mind, prompted by the ingenious animal brain, has found a way to exploit these ever-shifting spatial relationships so as to determine the real distances to objects relatively near to us. The principle of parallax is available to any human being who reflects on the fact that our stereoscopically paired eyeballs give us different angles on the world. If first one eye, and then the other, is covered, objects in the foreground of our vision will assume slightly different positions relative to objects in the background. The same principle operates at a slightly larger scale when driving down a road and viewing a tree in the middle distance against a mountain in the background: the image of the tree moves across the mountain in the direction opposite to the movement of the vehicle.
In an evolutionary sense, the use of two eyeballs to view
objects from slightly different angles is no accident. Unlike
prey animals whose eyes are typically oriented in opposite
directions to maximize their field of view, our eyes and those
of many other predators are set in the same plane because,
when focused on the same object, their different angles toward
that object enable the brain to calculate depth in the field
and accurately estimate the distance to the object.
Astronomers do the same thing at quite large scales. If a near star is observed against the field of much more distant stars, and then observed again six months later, its position will appear to have changed, because the earth's revolution around the sun has taken it to a new vantage point comparable to that of a second eye. Just as the brain processes the information gained from two slightly different angles to fix a distance, astronomers measure the angles generated by their lines of observation to mathematically calculate the distance to the near-field object.
Joyce's novel does not delve into scientific questions of
using angles to determine distance, but it does engage with
the underlying idea that objects may take on different
appearances based on the situation of the observer. The Greek
word παράλλαξις (parallaxis) means simply
"alteration," and the book offers many instances of the
appearances of objects being altered by the condition of the
In Lestrygonians, for example (the chapter in which the concept of parallax is introduced), food looks different to Bloom depending on whether he is in Burton's restaurant or Davy Byrne's pub. Lemon sole seems elegant in a fancy hotel, and "Still it's the same fish perhaps old Micky Hanlon of Moore street ripped the guts out of making money hand over fist finger in fishes' gills can't write his name on a cheque." It is characteristic of Bloom to flop back and forth in this way, seeing things first from one angle and then from another. Molly displays the same quality of mind, for example criticizing her husband for pandering to old Mrs. Riordan and then thinking, "still I like that in him polite to old women like that and waiters and beggars too."
In Cyclops, where the Homeric parallel turns the
denizens of Barney Kiernan's pub into one-eyed troglodytes,
Bloom's tendency to see things from multiple angles excites
the narrator's contempt: "I declare to my antimacassar
if you took up a straw from the bloody floor and if you said
to Bloom: Look at, Bloom. Do you see that straw?
That's a straw. Declare to my aunt he'd talk about it
for an hour so he would and talk steady." To the
reader, though, Bloom's inclination to look at subjects from
different points of view endows him with a kind of sterescopic
vision that marks him as a higher life form.
 In an application that is perhaps too large for the
scope of one of these notes, parallax may also describe the
bewildering variety of perspectives on the book's actions
taken by its many modes of narrative presentation. From the
first page, traditional third-person objective narration veers
into the subjective orbits of certain characters, both through
free indirect style and through interior monologue. At
times, notably throughout Stephen's Proteus and
Molly's Penelope, interior monologue can swamp
exterior narration, leaving the reader at the mercy of a
character's shifting thoughts, clutching at occasional
indications of what is "really" going on.
But external narration too proves to be a shifting, shifty thing. The newspaper headlines of Aeolus, the dramatic speeches and stage directions of Circe, the many parodic styles of Cyclops, Nausicaa, and Oxen of the Sun, the fragmented glimpses of scenes and individuals in Wandering Rocks, the irruption of first-person narration in Cyclops, the catechistic questions and answers of Ithaca: these and other narrative experiments shatter our expectation of a single, trustworthy perspective on the action. Ulysses does not opt, with Mulligan, for the view that we can know "only ideas and sensations" and therefore may dispense with searching for the truth. Its objects of representation and inquiry—e.g., individual lives in crisis, the fates of nations and races, the relations between human beings and animals—ask to be regarded as real entities demanding serious consideration. But those entities look different from different angles, like stars floating in multidimensional space and time.