What effect do the lights in the park have in the story?
The author devotes many words to describing the lighting of the park setting at night.
Half the enchantment of the place was due to the fact that it did not have the benefit of municipal lighting. The place was lit up by shop lights. One or two had hissing gaslights, some had naked flares stuck on poles, some were lit up by old cycle lamps, and one or two, like the astrologer's, managed without lights of their own. It was a bewildering criss-cross of light rays and moving shadows.
The lighting is very poor and irregular. It is noteworthy that the astrologer doesn't even have a light of his own. This description of the bad lighting helps the reader to imagine the scene, but more importantly it explains why Guru Nayak doesn't recognize the astrologer as the man he has been looking for. The lighting has become even worse by the time Guru Nayak shows up late that night.
The nuts vendor blew out his flare and rose to go home. This was a signal for the astrologer to bundle up too, since it left him in darkness except for a little shaft of green light which strayed in from somewhere and touched the ground before him.
Not only would Guru Nayak be unable to recognize the astrologer in the darkness, but the astrologer would have been unable to recognize him except for a lucky break.
The astrologer sent up a prayer to heaven as the other lit a cheroot. The astrologer caught a glimpse of his face by the matchlight.
At this point the author does not tell the reader that the astrologer recognizes his old enemy and nemesis Guru Nayak. So when the astrologer eventually calls the formidable man by his name, the reader is just as astonished as Guru Nayak. Not only does the astrologer seem to possess incredible supernatural knowledge, but he can even identify people in the dark. It never occurs to Guru Nayak that he might be talking to the very man he has been searching for. Even if he had been able to see the astrologer's face, he might not have recognized him because of the passage of time, because the astrologer had grown a beard and painted his forehead, because he was no longer an ignorant peasant but a smooth and articulate urbanite, and no doubt because Guru Nayak wouldn't understand how his illiterate enemy could have learned to be a "real" astrologer.
This is the hardest test the astrologer has had to pass since he adopted his profession. He distracts Guru Nayak by arguing about petty sums of money when his very life is in danger. Calling his nemesis by name is taking a great risk. Either Guru Nayak will suspect that the astrologer knows him personally, or else he will believe that the astrologer really and truly knows all things. Fortunately, he believes the latter and follows the astrologer's advice to return to his village and remain there for the rest of his life.