Schweiger's Hand Held Perimeter

And now for something completely different! We purchased this instrument at a specialist marine auction and it was catalogued as 'a Royal Navy gun sighting device'. Liking scientific instruments we secured the lot and with further online research we discovered auctioneers don't always know it all after all! We have now been able to identify it as not marine related at all but an early optometrists instrument invented in 1888 by Schweigger. It is a hand-held instrument and was pioneered for use at the patient's bedside as it was quite small, in contrast to the heavy clinical instruments that often filled huge crates. The instrument was set up with the vertical rod with the slanted plate placed against the lower rim of the eye socket of the eye being examined. The arc perimeter was then used to measure the angle of extent of the patient's visual field by moving targets along the arc until they can no longer be seen and the results recorded. The targets were coloured discs on the end of hand-held wands that the optometrists holds against the arc in various places. The arc revolves so that the field can be assessed across any meridian. The instrument can also be used to identify the characteristics of the visual field such as scotoma of which the physiological blind spot is an example that we all exhibit. These instruments continued to be used well into the twentieth century and assigning a date to this examples is very difficult, particularly as it carries has no makers marks or patent numbers, so we can only guess at its age. It appears to be in good used condition and would make a great addition to a specialist vintage optometrists collection or a brilliant entry in a 'guess its use' quiz! The arc measures 16" (40 cm).