In this episode, when they decide to test the "bright light" theory on Spock to see if it will kill the infection within him, they stick him in a chamber and shine incredibly intense light on him. Before doing so, McCoy says he'll get Spock some protective goggles to shield his eyes. Kirk vetoes this, saying that nobody on the planet will have goggles.

Yes, true... they won't have the benefit of eye protection, but why not even try to save the eyesight of the best science officer and first officer in the fleet??

That aside, this is actually a pretty good episode, which brings Season 1 of Star Trek to an end! We're a third of the way there! (Actually, a bit more than that, since Seasons 2 and 3 have just 26 and 24 episodes, respectively.) In order to rebuild my buffer a bit more, next week will be an interlude special, followed the week after by the first episode of Season 2.

Transcript

1
Captain's log, Stardate 3287.2. Spock has traced breakouts of mass insanity across this region of the Galaxy.
Spock: Here 500 years ago, this system 200 years ago, then here two years ago, in a straight line aimed at the Deneva colony.
Kirk: If only there was some sort of pattern...
2
Sulu: A Denevan ship is flying into the sun!
Kirk: Tractor beam!
Scotty: Out o' range, Cap'n.
Kirk: You didn't even look at a gauge or anything.
Scotty: Are ye questioning my leet Scottish engineering skillz?
3
Denevan pilot: {over radio} It's gone! I'm free! I'm—
Sulu: He's burnt up. He must have been... insane!
SFX: Drama!
McCoy: Jim, your brother Sam's family is on Deneva, right?
SFX: More drama!!
4
{on Deneva}
Kirk: Aurelan, what happened?!
Aurelan: They're here! They're here!
McCoy: Sam's dead, and your nephew's unconscious.
Kirk: Beam them up to sick bay, with no quarantine!
5
SFX: ZAP!
Spock: This creature should have been destroyed by our phasers.
Yeoman Zahra: It doesn't even look real.
Caption: Actual line!
Kirk: Yes, it looks like a novelty rubber vomit.
6
{one of the creatures attacks Spock}
Spock: Arrrrgghhh!!
Kirk: Spock!
7
{sick bay}
McCoy: It's injected tendrils through his nervous system. It controls people with intense pain.
Spock: It wants me to take over the ship, to take it to another planet. But I can control the pain.
Spock: Aarrrgghh!!
Spock: No, really.
Kirk: No. Stay in sick bay. With no guard.
8
{medical lab}
Caption: Spock breaks free and does SCIENCE
Spock: It's a one-celled creature, but of enormous size.
Kirk: Could we ever encounter an even bigger single-celled creature?
Spock: No, never.
9
Spock: The sun removed the infection from that Denevan pilot.
McCoy: But radiation, intense heat, nothing kills it!
Spock: We will have to nuke Deneva from orbit. It's the only way to be sure it won't infect other planets.
Kirk: No! I need a third option!
10
Spock: There is no other property of the sun which Dr McCoy or I can think to try.
Kirk: What about... bright light!
McCoy: Hey, we're the two smartest scientists on this ship.
11
{a bit later}
Spock: The bright light has cured me, but made me blind.
Chapel: Doctor, the lab tests show it was the UV which killed the creature!
McCoy: Oh my god! We exposed Spock to dangerous visible light when we could have used harmless ultraviolet!
12
{bridge}
Scotty: Bathing the planet in intense UV has cured everyone.
Spock: And my Vulcan physiology miraculously cured my blindness.
Kirk: All's well that ends well!
McCoy: Except your brother and his wife are dead.
Caption: And within 5 years everyone on Deneva develops melanoma...