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SIR W. DE W. ABNEY AND PROF. W. WATSON ON 
2. Threshold of Light. Extinction Curves: 
A series of experiments have been made .to determine the minimum intensity of 
light of the various colours which can be perceived when received on the fovea and 
at different distances from its centre up to 10 degrees. The chief difficulty in 
obtaining these measurements was to ensure correct fixation and to avoid retinal 
fatigue. Experiment showed that both these difficulties were very much reduced 
if the stimulus light in place of being continuously in action was only applied for 
a short time, sa 3 ^ about a second, with a comparatively long interval between 
flashes. 
With a Nernst light the arrangement employed is a slight modification of the arc 
colour-patch apparatus. It is shown diagrammatically in fig. 1. The source of light, 
a Nernst glower, A, is enclosed in a blackened metal box which has a small rectangular 
opening, the width of this opening being such that the glower is not screened from the 
collimator lens, C, and the height of the opening being such as to give a convenient 
width of spectrum. No collimator slit was employed, the glower itself acting as the 
slit and being placed at the principal focus of the lens, C. The dispersion train 
consists of two 60 prisms, D. A lens, E, forms a pure spectrum in the plane, FG, of 
a slide which carries a slit by means of which any required colour can be isolated. 
The position of this slit is read by means of a transparent scale attached to the slide, 
