Tom has to inch along the narrow ledge on the balls of his feet, with his face and body pressed against the building.
He simply did not permit himself to look down, though the compulsion to do so never left him; nor did he allow himself actually to think.
Most of us can identify with Tom. When we stand at the window of a tall building, or on the edge of a precipice, we can get overwhelmed by crazy ideas and fears and urges. It is hard not to think of falling, or jumping. We feel disoriented. This sensation is called vertigo, and some people are more susceptible than others. Alfred Hitchcock based his entire film Vertigo (1958) on this universal sensation.
When Tom finally gets to the precious sheet of paper, he still has a problem in picking it up, partly because he can't see it with his face pressed against the brick wall of the apartment building. He finally manages to get his fingers on a corner of the elusive paper.
At the same instant he saw, between his legs and far below, Lexington Avenue stretched out for miles ahead.
. . . a violent explosion of absolute terror roared through him.
Jack Finney handles the desccription very well. He has his viewpoint character facing the brickwall, unable to look down even if he wanted to. Then suddenly he sees Manhattan at night from that terrible perspective and fully realizes where he is and what a crazy thing he has done. He is paralyzed by horror. He has the paper but he can't move back towards his apartment window.
It was extemely likely, he knew, that he would faint, slump down along the wall, his face scraping, and then drop backward, a limp weight, out iinto nothing. And to save his life he concentrated on holding on to consciousness, drawing deliberate deep breaths of cold air into his lungs, fighting to keep his senses aware.
He tries shouting "Help!" but no one can hear him--and in a place like Manhattan no one would be likely to come to his aid even if they did hear him. He has never been so frightened or felt so lonely. This experience makes him realize the foolishness of his ambition to be "The Boy Wizard of Wholesale Groceries." He has ignored and neglected his wife, and now he is almost certain to lose his life over a piece of paper.
When he finally manages to break his window and crawl back into his apartment, he is a changed man. The reader feels that he will have a much better relationship with his wife in the future, and that both of them will be happier.
. . . he got out his topcoat and hat and, without waiting to put them on, opened the front door and stepped out, to go find his wife. He turned to pull the door closed and the warm air from the hall rushed through the narrow opening again. As he saw the yellow paper, the pencil flying, scooped off the desk and, unimpeded by the glassless window, sail out into the night and out of his life, Tom Benecke burst into laughter and then closed the door behind him.
Jack Finney is best remembered for his novel "The Bodysnatchers," which was made into the 1956 black-and-white horror film The Invasion of the Bodysnatchers about how extraterrestrial creatures are arriving on earth in "pods" and taking over people's brains.