THE THRESHOLD OF VISION FOR DIFFERENT COLOURED LIGHTS. 
93 
an image of this scale being projected by means of a lamp, M, and lens, P, on to a card 
fixed at 0 to a screen, OS, covered with black velvet. Attached to the slide, FG, is 
an annulus, Q, which has been described by one of us in a previous paper (‘ Phil. 
Trans.,’ A, 190, 156, 1897). This annulus consists of a uniformly increasing wedge 
of gelatine impregnated with finely divided lamp black. The annulus can be rotated 
by means of a milled head attached to a pinion which engages with a circular rack 
attached to the glass carrying the annulus. A graduated circle is also attached so 
that the part of the annulus opposite the slit can be identified. The light after 
passing through the annulus is received by a large lens, H, which forms an image of 
the front face of the prism train in the plane of the screen, OS. This image is 
received on a metal disc, I, which is covered with magnesium oxide. # The velvet- 
lined recess, T, is of such a depth that the light which misses the disc, I, and strikes 
the back of the recess and might be reflected from it, cannot reach the eye of the 
observer who is at L. Tunnels, R and Pd, lined with black velvet are used to screen 
off any stray light. Further, the Nernst light, prisms, &c., are all enclosed in boxes 
which are painted dead black inside. The instrument is placed in a room which can 
be rendered entirely dark. Small 3-candle power electric lamps, which are themselves 
screened and only switched on when required, are used to make the necessary readings 
of the annulus. These readings are made by an assistant who also keeps the current 
passing through the Nernst lamp constant. 
Between the Nernst glower and the collimator lens is placed a pivoted screen, B, 
which normally cuts off the light from the collimator lens. By pressing a key, M, 
placed near the observer’s hand, the screen, B, is raised by an electro-magnet. Thus 
the light only reaches the disc, I, when the observer presses the key, while by turning 
the milled head attached to the annulus he can alter the intensity of the light which 
falls on the disc, I. To give a fixation spot a rod of glass, J, is drawn down at the 
end to a diameter of about a millimetre and ground off at about 45 degrees. A small 
electric lamp, K, enclosed in a box with a small window covered with red glass, is 
placed opposite the end of this rod. The light from the lamp passes down the glass 
rod and owing to internal reflections does not escape from the sides till it reaches the 
ground end. At this end the light is scattered and thus gives a small source of red 
light. (By means of a resistance in the circuit of the lamp the intensity of this 
fixation light can be adjusted.) When fixation is desired for the central part of the 
retina, the circuits are so arranged that on depressing the key, M, the lamp, K, is 
extinguished. Thus the depression of the key by the observer causes the fixation 
light to disappear, and at the same instant causes the coloured light to illuminate 
the disc, I. When working away from the fovea the fixation light is disconnected 
from the key, M, so that the fixation spot does not vanish when the stimulus light 
is turned on. The reason for using a deep red fixation light is that such a light has 
little or no stimulus value for the rods. 
* Obtained by holding the disc over a piece of burning magnesium wire. 
O 2 
