Explanation for the appearance and disappearance of Edward Hyde SPOILER

SPOILER:

The movie can be confusing in the first third where Dr. Jekyll and Mr. Hyde keep rapidly replacing one another, almost in seconds it seems. In one scene there's Dr. Jekyll and in the next Hyde appears to wreak mayhem, havoc, and murder, then slips away and suddenly Jekyll is there.

I think I have the explanation. It came to me in the scene where Fanny Osbourne is spying on her fiancé, Dr. Henry Jekyll, in his laboratory. Before Jekyll fully undresses and steps into the chemical transformative bath, you see his eyes turn yellow, a sign of the personality transformation. I believe Dr. Jekyll had taken the chemical bath before, how many times I don't know. But the chemical bath's transformative effects were impermanent. Hence, Jekyll and Hyde were constantly alternating. As Hyde was the same height and build as Jekyll, the transformation could be quick, mostly in the face.

But once Jekyll took the chemical bath, his transformation into Hyde was more permanent. There was an antidote, however, the chemical compound, solicor, a fictitious chemical compound invented for the movie. Hyde swallowed the solicor to prove to Dr. Lanyon that Jekyll and Hyde were one and the same person.

Fanny immersed herself into the chemical bath to release her elemental, primal self but the only outward appearance was her yellow eyes. Jekyll felt he had no choice but to follow into the chemical bath.

At the movie's end the couple is transformed into what they both feel is their true, desired selves, man and woman following their passions and desires without inhibition.