Achromatic convex burning lens by Dollond in a mahogany frame. It was used by William Herschel in his study of the properties of 'radiant heat'. The lens and frame can be adjusted for inclination and are in a semi-circular brass ring mounted on a single column, mahogany stand. A brass collar and screw thread allows the lens mount to be adjusted for height and direction.

In observing the Sun, William Herschel tried a number of different colour filters to protect his eyes and found that the different colours seemed to block out different amounts of heat as well as light. In order to assess the relationship between heat and light he decided to test if heat showed any of the same properties as light by using this lens to focus the heat onto a thermometer. This experiment showed that heat, like light, could be refracted.