|View through the glass sphere of the Campbell-Stokes recorder to the green registration card, which was used in the past to record bright sunshine duration.
|The Campbell–Stokes recorder (sometimes called a Stokes sphere) is a type of sunshine recorder. It was invented by John Francis Campbell in 1853 and modified in 1879 by Sir George Gabriel Stokes. The original design by Campbell consisted of large ball lens set into a wooden bowl with the sun burning a trace on the bowl. Stokes's refinement was to make the housing out of metal and to have a card holder set behind the sphere. The unit is designed to record the hours of bright sunshine which will burn a hole through the card.
The crystal ball is typically 10 cm (4 inches) in diameter, and is designed to focus the rays from the sun onto a card mounted at the back and is set on a stand. The card is held in place by grooves of which there are three overlapping sets, to allow for the altitude of the sun during different seasons of the year. The recording of each day goes onto one card. In the northern hemisphere the winter cards are used from 15 October to 29 February, the equinox cards from 1 March to 11 April and 3 September to 14 October. The summer cards are therefore used from 12 April to 2 September. Each card is marked as to the hour, with local noon being in the centre, and is read in tenths
Source: Wikipedia contributors, "Campbell–Stokes recorder," Wikipedia, The Free Encyclopedia