The Heliometer was a split-image micrometer which fit on the objective end of a telescope. It was used to measure angles. It was pretty much the most advanced such instrument for measuring angles at the time of the 1761 and 1769 transits of Venus. Image courtesy of Specula astronomica minima (©Specula astronomica minima).

James Ferguson (1710-1776), FRS, a very skilled maker of astronomical instruments and an effective popularizer of the science, wrote several works explaining the phenomenon of the transit of Venus for those with an avocational taste for natural philosophy. He also gave advice on observing and harvesting of data, encouraging his readers to take an active part in studying the phenomena of nature themselves. The plate reproduced here is from our Archives' copy of Astronomy Explained Upon Sir Isaac Newton's Principles..., 6th ed. (London: W. Strahan, J. Rivington and Sons, et al., 1778).

Image courtesy of Specula astronomica minima (©Specula astronomica minima).

These are the tools (mathematical instruments) which were commonly used by Georgian astronomers to record their observations of the 1761 and 1769 transits of Venus, and by their early Victorian successors who illustrated the events afterwards. Starting from the upper left are a quill knife, a porte-crayon, a glass inkwell, a copper-alloy, leather, and glass travelling inkwell, a quill pen, an ebony and copper-alloy parallel rule, copper-alloy and steel dividers, a boxwood and copper-alloy English sector, and a boxwood plane scale.