From "Death by Black Hole and Other Cosmic Quandaries" by Neil DeGrasse Tyson

 

 

Lowell maintained that Venus sported a network of massive, mostly radial spokes (more canali) emanating from a central hub. The spokes he saw remained a puzzle. In fact nobody could ever confirm what he saw on either Mars or Venus. This didn't much bother other astronomers because everyone knew that Lowell's mountaintop observatory was one of the finest in the world. So if you weren't seeing Martian activity the way Percival was, it was surely because your telescope and your mountain were not as good as his.

 

Of course, even after telescopes got better, nobody could dupli­cate Lowell's findings. And the episode is today remembered as one where the urge to believe undermined the need to obtain accurate and responsible data. And curiously, it was not until the twenty-first century that anybody could explain what was going on at the Lowell Observatory.

 

An optometrist from Saint Paul, Minnesota, named Sherman Schultz wrote a letter in response to an article in the July 2002 issue of Sky and Telescope magazine. Schultz pointed out that the optical setup Lowell preferred for viewing the Venutian surface was similar to the gizmo used to examine the interior of patients' eyes. After seeking a couple of second opinions, the author estab­lished that what Lowell saw on Venus was actually the network of shadows cast on Lowell's own retina by his ocular blood vessels. When you compare Lowell's diagram of the spokes with a diagram of the eye, the two match up, canal for blood vessel. And when you combine the unfortunate fact that Lowell suffered from hypertension - which shows up clearly in the vessels of the eyeballs - with his will to believe, it's no surprise that he pegged Venus as well as Mars with teeming with intelligent, technologi­cally capable inhabitants.