Final answer:
Astronomers disagreed on the existence of canals on Mars. The straight lines observed by Percival Lowell were considered an optical illusion.
Step-by-step explanation:
The existence of canals on Mars remained a topic of serious dispute among astronomers. While the astronomer Percival Lowell claimed to see canals everywhere on Mars, many other observers could not see them at all and remained unconvinced of their existence.
When telescopes larger than Lowell's failed to confirm the presence of canals, it was generally accepted that the straight lines were an optical illusion caused by the human mind's tendency to see order in random features.