In the movie LOTR: The Fellowship of the Ring, Bilbo drops the One Ring on the floor and then leaves his house. Gandalf then goes to pick it up but just before he touches it, he sees an image of Sauron's Eye so he doesn't pick it up. A short time later, Frodo enters the house, sees the One Ring on the floor, picks it up and then shows it to Gandalf.
Gandalf then decides he has to travel to the city of Minas Tirith to do some research on the One Ring in order to find out if Bilbo's ring may be the One Ring.
Why was Gandalf unsure if Bilbo's ring was the One Ring if he had seen an image of Sauron's Eye when he was about to pick it up?