The History and Evolution of the Dissolve
Once a powerful storytelling device and pillar of continuity editing, the dissolve has become something of a ghost. This transition, which is also the earliest transition, has evolved from a tool for putting multi-shot films together, to a branch of film language that has a rich and powerful meaning all its own.
The dream sequence in Spellbound demonstrates two dissolves that have different cinematic meanings: one is used to indicate the entrance into an altered mental state (waking/dream state) and the other is used in a montage.
https://www.youtube.com/watch?v=JyPe1Jahyfo
Here's the iconic transformation scene from Metropolis, which demonstrates how dissolves can be used in SFX.
https://www.youtube.com/watch?v=JPYrepCGL-I
Another famous scene from Metropolis demonstrates this as well. Look for the dissolve from the podium to the pulpit. What kind of message was Fritz Lang trying to send to his audience with this dissolve?
Dissolves became a way for filmmakers to convey a deeper meaning, express passage of time, connect scenes, even a special effects technique. A dissolve was a slow and painless way to move the audience through each scene, but a dissolve could also signal an ellipsis of time -- a character pacing in a room waiting, a change of seasons, etc.
A screen-shot in the middle of a dissolve transition from Metropolis Dance scene.