If you want to watch the transit of Venus on June 5 - 6, just make sure you heed the advice of eye experts – don’t stare directly at the sun. And definitely don’t watch it if you're on LSD.

Anytime you look at the sun for longer than a few seconds it can lead to permanent eye damage. And that's what makes the Venus of transit — which won't occur again for more than 100 years — so dangerous: It gives people a reason to stare at the sun for long stretches. When unprotected eyes stare at the sun for a prolonged period of time, the intense visible light can damage, or even destroy, light-sensitive rod and cone cells inside of the retina, says David R. Copenhagen, a professor at the University of California's department of ophthalmology.

"Looking directly at the sun wipes out high resolution and some color vision," Copenhagen told Life's Little Mysteries. "About 10 minutes of looking at the sun can cause some degree of permanent blindness. Like brain cells, cones and rods don't regenerate — the amount that a person is born with is how much they'll have for the rest of their lives."

Now, you might ask, what dope would spend 10 minutes staring at the sun? Try someone on LSD. According to a 1973 study, "LSD produces a heightened degree of awareness of visual imagery. As a result, LSD users tend to be attracted to bright and colorful objects … It is known that LSD dilates the pupil, and this can result in a higher concentration of light being delivered to the macula."

The study focused on two cases, one of which was a 15-year-old girl who had heard a lecture at school about the dangers of taking hallucinogenic drugs and gazing at the sun. The patient thought that "it would be a neat thing to burn out my retinas," took LSD and stared at the sun for an unknown length of time. Although she was unable to read clearly after the event, she regained her full eyesight after two months.

So, how can you safely watch the transit of Venus? Pick up a pair of shade number 14 welder's glass, which can be bought at welding supply outlets and offer adequate protection. So does aluminized mylar that has been manufactured specifically for solar observation, according to NASA. Or use a shoebox to make a pinhole camera.