According to Grula, the man in question is Arthur S. King, a staff member at Mount Wilson from 1908 to 1943. He headed the observatory’s physics laboratory, where he specialized in divining the spectral lines of elements at various temperatures (important in discerning the chemistry of the heavens). He also showed how magnetic fields can affect the spectral line patterns, which helped scientists reveal the strength of magnetic fields in sunspots. He died in 1957 at the age of 81.
Grula even has a guess as to who “Mr. Forehead” is, the man seen peeking behind King. The “rimmed glasses,” “healthy shock of combed-back dark hair,” and “short stature,” according to Grula, suggest it might be Milton Humason, Hubble’s observing partner in surveying the expanding universe in the 1930s. The problem is that Humason is not seen in any other pictures taken that day, January 29, 1931, when Einstein visited the mountaintop. And in another picture of Humason and Hubble that I have seen, Humason is not that short; the top of his head comes up to about Hubble’s eye level. This man appears far smaller. So, one mystery remains.