This life work was more or less an injury and loss to me in many ways.
—Albert Abendschein, author of The Secret of the Old Masters, published in 1909
As such, I wouldn’t dare be the one to spoil Mr. Abendshein’s efforts by readily revealing the “secret.” It’s actually a fascinating read: the author spends most of the book walking you through his torment (via his many experiments in pursuit of the secret) until he gets to the end and reveals the discovered secret in a couple of paragraphs.
You can find a link to this book in PDF format in the Bibliography.
Below are the annotations I made while reading this book on the iPad: