

1905.] | On Colour- Vision by Very Weak Inght. 205: 
It would seem that after remaining in total darkness for two hours, a fairly » 
constant physiological condition is reached, and I have, therefore, compared : 
all the ratios with those obtained under these circumstances. 
It should be noted moreover, that the feebler the sensation the less the: 
judgment of the observer enters into the result, until finally, when both bands: 
are so faint that they can only just be glimpsed from time to time, the 
sensations may be considered equal if each is seen with equal difficulty. 
_ In order to record a series of observations made during a long period of 
darkness, I devised the simple arrangement shown in Fig. 3. 3 
Fia. 3. 

The divided circle is fixed about 5 mm. in front of a larger square plate on 
which two thin pieces of wood A and B, are pinned by the pivots C and D. 
Each piece is furnished with a notched sector of sheet brass in which the 
spring catches H and K engage. The brass plate E fixed to the tube 
containing the Nicol prism carries, in addition to the verniers, two thin springy 
strips of sheet brass F and G. To the end of each strip is soldered a small 
drawing pin carefully sharpened, and a thin card is fastened to each of the 
pieces A and B. : | | 
After the first observation made in the dark room the strip F is pressed 
so that the point marks the card. A is then moved one notch so that the 
mark for the next observation may come on a different part of the card. I 
have found it convenient to use the other spring G for recording results 
