The bulk of RGO 66 comprises the incoming and copy correspondence of the chief assistant, later superintendant, George Mathews Whipple during 1871-5. Much of it is with the chemist and astronomer Warren De La Rue whose photoheliograph was moved to and used at Kew in the early 1870s. The correspondence is largely concerned with solar photographs and the practicalities of their production. There are a few financial papers and some records of solar and lunar observations made both at Kew and elsewhere.