Einstein's Dissertation of 1905
on Molecular Dimensions
31. Expert opinion by Alfred Kleiner and Heinrich Burkhardt on Einstein's Dissertation
Zurich, 22 July 1905
(Kleiner:) The arguments and calculations to be carried out are among the more difficult ones in hydrodynamics.(Burckhardt:) What I checked, I found to be correct without exception.
Footnote  to Letter 31 in Collected Papers vol. 5: Both Burckhardt and Kleiner overlooked a serious error in Einstein's calculations. A major discrepancy between experimental results obtained by a researcher in 1910 and Einstein's theoretical prediction led Einstein to investigate whether he had made a calculational error. After he had unsuccessfully tried to find it himself, he turned to a collaborator, who discovered the error.
239. TO LUDWIG HOPF
Einstein asks Hopf to check his calculations
Zurich, 27 December 1910
Dear Mr. Hopf,
I thank you once again for the splendid toys you sent for my son.I rechecked my old calculations and arguments and could find no errors in them. You would do a great service to the cause if you made a thorough examination of my argument. Either there is an error in my work, or Perrin's suspended substance has a larger volume in the suspended state than Perrin believes.