180 Mr. G. Johnstone Stoney [Feb. 6, 



man endeavours to ascertain. His first inquiry is as to what has 

 occurred in the outer world. This in the case we have taken he finds 

 to be motions with ascertainable periodic times within and between 

 molecules of the gerauium, followed up by an undulation around that 

 has the same periodic time as those motions, a very small part of 

 which reaches the black pigment of the eye through the pupil. This 

 is the first or z part of a scientific explanation of the phenomenon. It 

 is the explanation of what is going on in the outer world, and the z 

 of the problem would be considered by the scientific man as having 

 been fully discovered if these motions and all that relates to the laws 

 under which they occur had been made out. 



The next, or y part of the investigation, is to discover what change 

 this z produces within our organs of sense on those parts of them 

 which stand in a direct functional relation with the associated 

 nerves. This, in the example we have taken, would consist in ascer- 

 taining — 1st. What new substances ay)pear in consequence of the 

 (probably three) photographic effects which the incident light has on 

 the pigmentum nigrum ; 2nd. What effect these have upon the appa- 

 ratus of rods and cones which keeps tapping against the pigmentiun 

 nigrum; 3rd. What is the connection between this last change and the 

 excitation of the nerve. The next in order would bo the x part of 

 the inquiry. It would be an inquiry into the j^hysical change which 

 the presence of these substances produces in the retina and along the 

 optic nerve. If all these, and all about them, were fully made out 

 the y and x parts of the explanation would be comj^lete. And, finally, 

 if it could be discovered what physical change ensues in the brain 

 itself the lo part of the problem would be solved, and the whole 

 scientific exj)lanation would be then complete. 



In order of time the z part of the phenomenon first occurs in the 

 outer world. Some excessively small part of this gains access to our 

 organs of sense and produces in them the y part of the phenomenon, 

 which acting on the nerves, and developing in them the x part of the 

 phenomenon, excites them in turn to make that stir within the brain 

 which is the iv part of the phenomenon, along with which the a part of 

 the phenomenon presents itself and the percej^tion of green at certain 

 situations in space comes to form for the time a part of our mind. 



We are, however, as yet only concerned with the z part of the 

 phenomenon — that part which takes place in the outer world, and 

 with which science has been up to the present able most fully to 

 deal. As we have seen, it consists entirely in motions. It has been 

 found possible to measure many of these motions in various ways ; 

 and, to give some notion of what the universe really is, I will state 

 the results of some of these determinations. 



Metrets. 



Scientific measures are now made in metres and divisions of the 

 metre. The metre itself is a few inches more than a yard long, and 

 is divided into metrets, a convenient name for its decimal subdivi- 



