In 1769 Cook looked through a telescope to time the transit of venus across the sun.
We all know that you should not look directly into the sun.
Did they have sun glasses way back then?
I think people used smoked glass to peer through, in the olden days.
Did he look into a telescope? The technique of focussing the instrument to display its image on a piece of card for safe observation had been known for at least a century and a half before his time; it’s also the safe way to view eclipses.
The answer, perhaps surprisingly, is that nobody quite knows.
Wayne Orchiston is the person who’s written about Cook’s Tahitian astronomy in recent years and this paper (a pdf) is the most detailed one I’ve seen by him on the technical details of the observations. The question of filters versus projection is raised in the discussion section at the end - make sure you read to the end since other questioners return to the matter.
He’s pretty open about the question having puzzled him. As he points out, the written accounts - see this website for the journals kept by Cook and other crewmembers - simply omit the relevant details. As for visual sources, there were good diagrams published of the sites chosen and some of the observing set-ups, but there’s nothing convincing as a contempory view of any of their telescopes in situ.
The general tenor of the discussion following his paper is that eyepiece filters were historically far more commonplace in solar observing that we’d possibly contemplate these days. (The danger is that the telescope concentrates the rays on the filter, so they can shatter and suddenly expose the eye to the concentrated glare.)
One piece of evidence missed in the linked discussion is that Delisle discussed the use of filters in the influential advice for observers he published and widely distributed ahead of the 1761 transit. (I haven’t seen the original text, but the detail is mentioned by Harry Woolf in his The Transits of Venus, Princeton, 1959, p62, still the standard work on the 18th century transits.)
While Cook, Banks and the others on Tahiti in 1769 certainly could have observed the transit by safely projecting the Sun’s image, I’d tend to guess in the direction of them having used filters.
Of course, it’s also possible to look at the Sun through a telescope with no eye protection, like Galileo did. Of course, Galileo also went blind, so it’s not exactly a good idea.
bonzer’s post is one of the reasons I love this place. No matter how obscure the question, someone here knows all about it. 
Thanks, silenus.
On Galileo’s methods for looking at the Sun and the hazards to his eyes, this page goes into the matter in exhaustive detail. Prior to using the projection arrangement, he probably only looked at the Sun through his telescopes when it was dimmed by clouds or at sunset. And his much-later blindness was probably unrelated. Though the page does mention the cases of astronomers, including Newton, who did suffer eye damage by looking at the Sun.
Not a method available to Cook, since part of the whole point of trailing out to Tahiti was to have clear skies and to be able to follow the transit over the entire six hours of it through the day.