"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 stereoptically 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 views 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 subject. 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." But to the reader Bloom's inclination to look at subjects from different points of view endows him with a kind of stereoptic vision that marks him as a higher life form.