A letter from William H. Mumler to J. Burns printed in the July 1st, 1872 edition of The Harbinger of Light. The letter describes to Burns that Mumler cannot guarantee a specific spirit will appear as he just acts as “a medium for preparing and developing the negative”.
Mumler goes on to state which evidence that spirit photography is true, “12 years of accumulative evidence”. He also states he has the evidence of 1000s of people who had photographs taken and recognized spirits within.
Mumler goes on to give specific instances of people who had their photograph taken and recognized the spirit. He states that he believes he has given enough evidence that spirit photographs are true and that the “golden gates between this and the spirit world are really ajar”.