Name: Dr. Marko
Other Names: Dr. Marco
First Appearance: Detective Comics (vol. 1) #44 (Oct. 1940): "The Land Behind the Light"
Number of Appearances: 1
Note: invented device to transport people to Fourth Dimension
Dr. Marko invented a machine that allowed him to transport people to the "Fourth Dimension." Batman and Robin visited Dr. Marko, who showed him his machine. Dr. Marko transported himself into the Fourth Dimension using his machine. Batman and Robin followed him there. They found that the "Fourth Dimension" was a fantasy world populated by giants and tiny people.
At the end of the story, Robin wakes up. He was only dreaming about his adventures in the Fourth Dimension. Everything that happened there, as well as Batman and Robin's visit to Dr. Marko, had been a dream prompted by Robin's bedtime reading of a book titled Giants and Dwarfs in Myth and Fable.
In the context of this story, the eccentric "Dr. Marko" was probably not even a "real" person. He was simply part of Robin's dream.
Note that in most instances, this character's name is spelled "Marko." But the first time his name is mentioned, by Batman in panel 2 on page 2, his name is spelled "Dr. Marco."