38a SCIENCE PROGRESS 



free from the centres to which they are attached, the change 

 occurring is to be identified with the bleaching- of the visual 

 purple. 



Sir William Abney ' likens the colour-perceiving apparatus 

 in the retina to three pendulums, one for each primary colour. 

 Thus he would regard Ives' curve as due to the sum of three 

 curves, one representing the amount of energy taken up by 

 the mechanism that produces the red sensation, one representing 

 the amount of energy taken up by the mechanism that pro- 

 duces the green sensation, and one representing the energy 

 taken up by the mechanism that produces the blue sensation. 

 We shall leave the question as to whether there is only one 

 or whether there are three classes of vibrators in the retina 

 open at present, and shall proceed to a further study of experi- 

 mental results obtained with the apparatus shown in fig. i. 



^A W VAPAm . 



Fte. 4. Fig. 5. 



4. A clockwork kymograph — an apparatus consisting of 

 two drums over which a band of smoked paper passed with a 

 uniform speed — was lent me by the Physiology Department, 

 and I endeavoured to obtain records of the motion of the simple 

 pendulum. They did not prove very successful as long as a 

 single thread was employed, but when the simple pendulum 

 was replaced by a short brass cylinder suspended by two 

 threads GH and EF, one at each end, and a short thread HP 

 was allowed to trail a metal point P on the smoked paper, which 

 was passing in the direction shown by the arrow, no difficult}' 

 was experienced. Fig. 5 gives two records, both, of course, 

 reduced in size. The original was in each case about 24 in. 

 long. For the upper curve the period of the compound pen- 

 dulum was 1-93 sec., and for the lower curve it was 2*105 

 sec. In each case the period of the simple bifilar pendulum 



1 Tyndali Lectures on Colour Vision, pp. 36-9. 



