A very rare and intriguing Dobbie-McInnes Hopkinson Flashlight Indicator with Photographic / Telescopic Hood. This particular engine indicator is particularly rare given the condition of the photographic hood which has survived well over one-hundred years. According to John Walter in his 'Engine Indicators: Autographic and other recorders from the time of James Watt to Today (Part 5: Clever Tricks)' (2024), Dobbie McInnes may have only made 250 Hopkinson Optical Indicators, so the likelihood of others surviving in a condition as complete as this one is highly unlikely.
The design of the Hopkinson Indicator is such that a pin-point beam of light is directed onto a mirror and reflected on a photographic plate or through a special observation telescope, where the changes of pressure during the operating cycle of an engine are traced as a permanent record.
This one is in remarkably good condition. Two screws are missing from the hood and there is a small chip the corner of the glass plate but it is structurally good. A fantastic, collectors' indicator which also makes for a great display item.
