Rittenhouse Beats Fraunhofer!

“By pursuing these experiments it is probable that new and interesting discoveries may be made respecting the properties of this wonderful substance, light….but want of leisure obliges me to quit the subject for the present… ” David Rittenhouse in a letter to Francis Hopkinson (1786).

“Want of leisure?” That is certainly understandable! Rittenhouse was the Treasurer of Pennsylvania and had other duties in the newly-formed USA. He would become the first Director of the US Mint in 1792. Hopkinson, a signer of the Declaration of Independence, caused Rittenhouse’s temporary loss of leisure when he sent him the initial letter (March 16, 1785) describing what he saw (diffraction) while looking through a silk handkerchief. But should Rittenhouse have taken the time off from his work on diffraction gratings?

David Rittenhouse

“It is difficult to point to another single device that has brought more
important experimental information to every field of science than the
diffraction grating. The physicist, the astronomer, the chemist, the
biologist, the metallurgist, all use it as a routine tool of unsurpassed
accuracy and precision, as a detector of atomic species to determine the characteristics of heavenly bodies and the presence of atmospheres in the planets, to study the structures of molecules and atoms, and to obtain a thousand and one items of information without which modern science would be greatly handicapped.” (J. Strong, J. Opt. Soc. Am. 50 (1148-1152), quoting G. R. Harrison), from Diffraction Grating Handbook, 5th edition, Christopher Palmer, Thermo RGL.

Big Diffraction Gratings at LLNL
NASA IRIS Telescope with Imaging Spectrograph, with grating, first light July 2013

It’s tough to say who discovered/invented the diffraction grating. Isaac Newton wrote in Opticks about “scratches made in polished plates of glass.” A contemporary of Newton, James Gregory, studied diffraction patterns produced by bird feathers. However, it looks like Rittenhouse in 1785/86 was the first to build and use a diffraction grating to make spectral measurements. Rittenhouse said, “…I made a square of parallel hairs about half an inch each way. And to have them nearly parallel and equidistant, I got a watchmaker to cut a very fine screw on two pieces of small brass wire. In the threads of these screws, 106 of which made one inch, the hairs were laid 50 or 60 in number…” That was one year before Fraunhofer’s birth and three decades before his experiments. But Fraunhofer did not take leisure; he finished the job!

Joseph von Fraunhofer

For an excellent summary of Rittenhouse’s experiments and results, see I. D. Bagbaya, “On the History of the Diffraction Grating,” Soviet Physics Uspekhi Vol. 15 No. 5, p. 660-661, March-April 1973.