Prof. Minchin's Experiments in Photoelectricity. 235 



tion of a really scientific photometer. It cannot be said that 

 the comparison of two lights by means of a spot of grease is 

 a very satisfactory procedure ; but in the seleno-aluminium 

 cell we have, on account of its great sensitiveness and range 

 in the spectrum, an approximation to the possibility of an 

 electrophotometer — at least so far as the comparison of lights 

 is concerned. For, assuming that, by means of such a cell, 

 we take, as it were, each light to pieces, and note the inten- 

 sities of the different colours — which are directly proportional 

 to the squares of the corresponding electromotive forces — 

 we could then apply the principle of Newton's chromatic 

 circle to determine the value of the resultant light. The 

 second problem — and that which in 1877 induced me to 

 work at this subject — is the electrical transmission of an 

 image to any distance ; in other words, the construction of a 

 telephotograph. The problem seems to be one of exceedingly 

 great difficulty — much more difficult than the problem of the 

 telephone — because the parts of an image are simultaneous 

 and not, like the sounds of the voice, successive ; and, indeed, 

 we have also to deal with a quick succession if we are to 

 transmit a living moving image, such as Homer depicted on 

 the shield of Achilles. The early attempt which I made con- 

 sisted in the construction of a cable, somewhat on the model 

 of the optic nerve. The optic nerve consists of a bundle of 

 fibres, each a conductor of electricity and each separated 

 from its neighbour by being surrounded by a medium. One 

 set of ends of this nerve abuts on the retina, which is its 

 sensitized plate, and the other in the brain. It is well known 

 that light incident on the eye causes a photographic decom- 

 position on the retina ; and I believe that images have 

 been seen on the retina of a rabbit which was immediately 

 killed after a strong light had been presented to its eye. 

 The images, then, of external objects are transmitted along 

 the optic-nerve cable to the brain, where b}^ some means 

 or other they result in a process of thought. Possibly 

 thought is an equivalent of at least a part of the originally 

 incident energy. No satisfactory solution of this problem 

 will be attained by any slow and painful mechanical pro- 

 cess of tracing out in succession the various portions of a 

 picture ; and it does not now seem that we are near any 

 true solution of the problem, whatever startling stories the 

 newspapers may from time to time report. 



The third problem is the direct transformation of the 

 radiant energy of the sun into work useful to us, without the 

 consumption (at least on any large scale) of materials on the 

 earth — in other words to get rid of that terrible waster of 



