Doc uses a magnifying glass in 1955 to read the letter written by his 1985 counterpart.

" He [Doc] pulled one more thing out of his pocket. / "Look at this pocket with the magnifying glass," he told Marty. / Marty took the handle of the glass from Doc and held the lens over the photo. There, sticking out of Biff's pocket, was the top half of the Sports Almanac! "
—From Back to the Future Part II by Craig Shaw Gardner (quote, page 126)

A magnifying glass was a convex lens set in a round frame with a handle, which could produce a magnified image of any object it was held over.


Doc looks at time control circuit

Doc examines the burned-out time circuit control microchip with a magnifying glass.

Dr. Emmett Brown had one of these magnifiers, which he loaned to Marty McFly in 1985A so that Marty could examine the photograph accompanying the newspaper story about Biff Tannen winning his first million on a horse race back in 1958. It was while doing so that Marty spotted the Grays Sports Almanac sticking out of Biff's pocket.

The younger Doc in 1955 used a magnifying glass on November 13 to enable him to read aloud the letter from his 1985 counterpart, written in 1885 and delivered to Marty by Western Union — as well as to study the enclosed map for reaching the DeLorean time machine hidden in the Delgado Mine; the repair instructions and the schematic diagram on how to to fix the DeLorean using 1955 components; and the burned-out time circuit control microchip.

Stuck in 1986G, Marty urged First Citizen Brown to look closely at his photo from 1931 in hopes that his claim as being a time traveler was true. Brown used a magnifying glass to look at the photo and noticed Marty, as well as an alternate version of himself.


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