Board of Trade Lantern Oil Lamp
The picture to the left shows part of one of the original Board of Trade Lanterns (perhaps no more than 20 would ever have been made. We have one; the National Maritime Museum in Greenwich has another).
Developed as a result of the Parliamentary Committee which reported in 1912 it aimed to reproduce typical ships' lights at a certain distance. The patient viewed it in a darkened room sitting six metres away. The lantern was superseded in 1970 but the arrangement and dimensions of its two half millimetre 'point sources' persists. The lantern requires a chimney on account of the oil lamp contained within.
This picture to the left shows the (Vertical Pattern) Edridge-Green Lantern, a funnel-shaped colour perception test lantern (Vertical pattern) with rotating colour discs, fitted for electrical illumination. The original lantern was designed circa 1910 with both vertical and horizontal options, for use with an electric or oil lamp. The original manufacturers were William Gowlland Ltd but this object is a later model by Rayner. Crucially its readings can be taken independently of the colour vision of the examiner.
The Giles-Archer Colour Unit, designed in 1935, was a black metal cylinder with attached revolving lens disc featuring five coloured lenses and one aperture. A sliding bar behind the lens disc contained a further three apertures. The internal bulb holder held an Osram 15 watt pigmy bulb and the whole unit would have been plugged in to a lamp bracket for use. It was designed by George Giles (who subsequently became Secretary of the British Optical Association) and based on the existing lamp by Edridge-Green.
There was a special 'Aviation Model' of the Giles-Archer Colour Unit. Essentially a practical instrument, it helped to identify colour deficiency with regard to the signal colours an aviator might be expected to identify during day or night time flying. In the 1930s many ophthalmic opticians were also being approached by a new breed of amateur aviator, anxious to know if they would stand a chance of passing their flying examinations. As its principle purpose was to distinguish between ‘dangerous’ and ‘non-dangerous’ deficiencies it was designed to assess border-line cases of Anomalous Trichromatism or Dichromatism rather than to confirm Normal Trichromatic vision. Mistaking white for yellow or blue for green did not necessarily result in a fail. A patient was allowed fifteen minutes to adapt to the dark in the testing room. Single coloured lights were then shown to him from a distance of six metres, employing successive contrast. The varying apertures in the sliding bar allowed the light to be shown as if emanating from a greater distance. Another model catered for the test needs of marine navigators and railwaymen.
Colour vision test instrument
Although it is not a lantern the instrument to the right is not dissimilar in principle to the Giles-Archer/RAF test and shows close affinities to Scripture's Colour Sense Tester of which it may in fact be a variant. It consists of a fixed white metal disc containing three filters, (clear, blue and green), in front of which revolves a second disc with a milled edge, containing twelve apertures labelled A to L. (i.e. several coloured glass filters are missing). It was for testing depth of vision. The catalogue for Stoelting of Chicago states explicity that Scripture's Test was intended to replace 'the somewhat cumbersome' Edridge-Green lantern for use with soldiers, sailors and railroad employees. We know of one marked example of Scripture's test supplied in Great Britain by Thomas Armstrong of Manchester.
The Sloan Colour Threshold Tester (CTT) was an American colour vision test lantern consisting of a black wooden box with leather carrying handle. Hinged doors at each end provided access to two rotating discs of 8 filters and two control knobs. There was an internal 60 watt lamp. In essence this is a simple colour perception lantern run from Mains electricity and lit by a relatively simple opal white tungsten lamp with no provision for stabilisation. Dr Louise L Sloan of the Johns Hopkins Hospital (Wilmer Institute) in Baltimore introduced the device in 1944. She aimed to show progressive intensities of the coloured filters in her lantern. This approach was intended to quantify the degree of any deficient colour perception, as a means of judging likely competence in practical situations, notably for American Air Force personnel. Initially the patient was shown colours at the highest intensity. An unusual feature was the use of a small blue 'orientation' light on each side of the test light display. This particular specimen, which was given to the British Professor R. J. Fletcher in Baltimore in 1967 has since suffered deterioration of the Wratten (Kodak) filters.
To the left is an unidentified lantern of part cast, part machined metal construction, covered in black crackle paint. Discovered and donated to us by the Maritime and Coastguard Agency, it has an oval front with rounded ends on a small sloping pedestal with two central apertures. The rear comprises two knurled handles for adjusting coloured filter discs and two levers for adjusting the size of the apertures. It also features a corrugated tubular lamp housing with brass bulb holder and two observation windows glazed with lenses. We think the instrument must either be a single prototype or one of a small batch and would love to hear from anyone who knows better.
Hand-held colour vision test
Meanwhile, here on the right is another lantern, this time a hand-held colour vision test. It has a white metal cylindrical construction with two milled-edge adjusting wheels for colour filters and a black rubber light-excluding eyecup. There is a screw hole for a possible stand attachment. The object lacks any nameplate or maker's details so may have been a prototype developed for seafarers. It came to us via the Department of Transport, Local Government and the Regions (DTLR).
Holmes Wright Lantern
Finally we bring the story up to 1974 and the Holmes Wright Colour Perception Lantern. The item pictured is the Type 'B', the marine model intended for the merchant navy (as opposed to Type A, the armed services and civil aviation model) manufactured by M. S. Precision Products Ltd. W. D. Wright, of Imperial College, was asked to design a new lantern in 1970 at the behest of the Air Ministry in order to replace the aged Board of Trade Lantern. The presentation of the test lights in the Type B lantern was deliberately designed to reproduce that of the Board of Trade Lantern, so that the continuity of the standard of acceptance would not be broken. The Type A Lantern, however, had some differences. The initial production began in 1974.
A short history and technical features are to be found in Holmes, J. G. and Wright, W. D., 1982, 'A New Colour Perception Lantern', Colour Research and Application 7 (2) Part 1, pp.82-8.
Maybe 400 or so were produced in total and these went all round the world. As of 2007 examples were known to still survive in Oslo and Melbourne. The test was designed to be used at 6 metres (accomplished by viewing in a mirror at 3 metres in a darkened room). It has a revolving disc for changing the colour setting (to red, dark red, blue green, yellow green or white), an adjusting lever for large (5mm) or small (0.5mm) diameter lights and a letter-box aperture for projection of the lights.