1894. NOTES AND COMMENTS. 413 



another, and so became more closely packed. When closely packed 

 they allowed a current to pass through more readily. The arrange- 

 ment is termed a " coherer," on account of the capacity of the con- 

 stituent particles to become more closely packed. After coherence 

 the original looseness and consequent resistance may be restored by 

 tapping. Professor Lodge's " coherer " consisted of iron borings 

 loosely placed in a glass tube. The glass tube was placed in a large 

 copper " hat " to protect it from electrical influences other than the 

 radiations. The radiator was cased so that its radiations were 

 directed to the coherer, and the coherer was read by a reflecting 

 galvanometer with which it was placed in circuit. 



With these appliances there were shown : — The influence of the 

 radiations on the conducting power of the coherer; the polarisation of 

 the radiations by gratings of parallel copper wires, the vibrations in 

 the direction of the length of the wires being arrested, the others 

 passing on; the reflection of the radiations from bright metal surfaces, 

 from wood, and from the human body ; the transparency of glass to 

 these radiations, and the refraction of the radiations by means of a 

 prism of parafiin : all indicating the analogy of these radiations to 

 those of light. 



From such experiments came Professor Lodge's theory of vision, 

 which, however, he put forward as the suggestion of a theory rather 

 than its promulgation. Briefly put, his suggestion is that light may 

 excite transverse vibrations in the rods and cones of the retina, that 

 these vibrations may alter the resistance of some part of the eye and 

 so cause the recording of the light stimulus, and that the stimulus 

 corresponding to the process of tapping for restoring the coherer to 

 its original state would be the influence of darkness. Between the 

 stimulus of light and that of darkness there would be persistence of 

 vision. These ideas were illustrated by means of the coherer, the 

 restorative tapping being effected automatically through vanes 

 revolving by clockwork. 



In the subsequent discussion. Lord Rayleigh insisted on the 

 necessity of the three-fold nature of colour-vision being taken into 

 account in any theory of vision ; Professors Armstrong and Fitz- 

 gerald naturally laid stress upon the physico-chemical side of vision 

 and the resemblance of the retina to a sensitised plate ; while Pro- 

 fessors Rutherford and Schaefer discussed the anatomical structure 

 of the eye as a coherer. But Professor Burdon Sanderson made 

 what, in our view, was at once the most obvious and the most valu- 

 able criticism, when he pointed out that, while Professor Lodge's 

 suggestion was worthy of consideration as a theory of the action of 

 light upon the retina, it was not a theory of vision at all. 



Mind and Body. 

 The point raised by Dr. Burdon Sanderson in connection with Pro- 

 fessor Lodge's suggested theory of vision raises the perennial difficulty 



