HAIRLINE • DICCIONARIO INGLES.COM • Definition of Hairline

... Rip sighted past the vertical hairline at the belt light. "That's a little too far. Take a small step to the right. Good ... just a few inches more ... hold it. You're right in position. Stand where ...

— Rip Foster Rides the Gray Planet • Blake Savage

... and shook his head disgustedly. The scanner screen was still dark. Having adjusted the delicate mechanism to eliminate the white flashes of static, he couldn't find them again. He sat back in his chair for a moment, mopping his brow and watching the white hairline in its continuous swing around the face of the scope. As the line swept to the top of the screen, he saw the blip outline of a jet boat and recognized it as one belonging to the Polaris. Then, slowly, the line swept down and Roger suddenly saw the blip outline of a second craft. ...

— On the Trail of the Space Pirates • Carey Rockwell