Answer: Midas is an allusion to King Midas, who could turn everything he touched to gold. In the Great Gatsby it was used to symbolize the greed and materialism of the characters.
Explanation: "I bought a dozen volumes on banking and credit and investment securities, and they stood on my shelf in red and gold like new money from the mint, promising to unfold the shining secrets that only Midas and Morgan and Mæcenas knew."