History isn't fair. Bob Wickersham had the misfortune to animate at Disney in the 1930's, when there were no screen credits. In the 1940s he directed at Columbia on series like The Fox and the Crow, but those cartoons were never highly visible on TV, VHS or DVD. According to Alberto Becattini, Wickersham has a pretty good filmography as a Disney animator. He worked on The Band Concert, Pluto's Judgment Day, Lonesome Ghosts, The Flying Mouse, Who Killed Cock Robin?, The Old Mill, Little Hiawatha and Wynken Blynken and Nod. It must have been tough to work on the cream of the crop with no public acknowledgment whatsoever.
In Thru the Mirror, Wickersham gets the entire opening of the cartoon, setting up the situation and taking Mickey through the mirror, up through jumping rope on the telephone cord. Wickersham knew how to draw appealing poses. His drawings don't have the same strong rhythm as Fred Moore's, but the proportions are very pleasing and the poses are well balanced. Wickersham's Mickey is also extremely flexible and he has a functioning brain. If you keep your eye on Mickey during these opening scenes, you see that Mickey reacts to everything that happens to him in a variety of subtle ways.
When the animation passes to Dick Lundy, Lundy's proportions are not as appealing as Wickersham's; Lundy draws Mickey's eyes, nose and ears somewhat smaller. When it comes to animation, though, Lundy is as good as anyone in this cartoon. He does three dance sequences: one on the top hat, one with the gloves and one with the Queen (a Garbo caricature). Each dance is completely different from the others. Animating dance is difficult enough, but Lundy had to master three different styles all for a single cartoon. That's a real achievement.
Leonard Sebring is less well known than Wickersham. According to Becattini, he only worked in animation from 1933-36 and only at Disney. I wish I knew why he left the field. While his dance animation isn't as sophisticated at Lundy's, he meets the challenge of dealing with a deck of cards and matching a musical beat. These scenes are not simple by any means and Sebring handles them flawlessly.
Hardie Gramatky is best known as a childrens' book author and illustrator. He created Little Toot as a book, which Disney adapted to animation in Melody Time. Is there a case where anybody else left Disney and created something that the Disney studio then animated? I can't think of any.
Gramatky's animation isn't as controlled as the other animators. He doesn't have a good sense of weight and his animation feels a little twitchy, as if he didn't understand spacing fully. His drawing of Mickey is also the crudest in this cartoon. Mickey's exit in shot 33 feels like he's being pulled out of the scene rather then moving under his own power. I don't think that it's any mistake that Gramatky was given the broadest scenes to animate, where his looseness would be least noticeable to the audience.
Wickersham handles the end of the cartoon, and his scene of Mickey growing back to normal size is done completely differently than the shrinking scene he animated early in the cartoon. Like Lundy doing dances, the Disney animators of the time had multiple solutions to any problem and didn't need to repeat themselves.
This cartoon, like many mid-'30s Disney cartoons, gets by on charm. The story is slight and there's not much acting. However, there's no shortage of gags or imagination and each idea is so well timed and executed that it captivates the audience. There's a level of skill and confidence in this cartoon that leaves most other studios of the period in the dust.