1885.] on how Thought present itself in Nature. 181 



sions, that is subdivisions each of which is the tenth part of the one 

 before it in the series, and ten times the next after it. The decimetre, 

 or tenth part of a metre, is the first of these metrets ; it is about a 

 hand-breadth. The next metret is the centimetre, the hundredth part 

 of a metre, and is about a nail-breadth. The third-metret is the 

 millimetre, about the distance across a small pin's head. The fourth- 

 metret is the tenth of this, and is about the thickness of a sheet of 

 paper. The tifth-metret is microscopical ; it is intermediate in size 

 between the diameters of the red and white disks that float in human 

 blood. The tenth of this, the sixth-metret, would be a very small 

 object in the microscope, and no microscope is able to show the 

 seventh-metret, which is the next of the series. However the study 

 of nature has obliged us to go farther than the microscope can pene- 

 trate, and leads us on to, at all events, the tenth and eleventh of this 

 series of metrets. The tenth-metret is so small that a child during 

 the years of its most vigorous growth is growing at the averao^e rate 

 of between thirty and forty of them every second, and the eleventh- 

 metret is the tenth part of this again. This is about as far as the 

 scientific examination of nature has as yet obliged us to go. 



Quantitative Determinations — Light and Sound. 



Now the waves of light as they travel between the objects we are 

 looking at and our eye, are of various lengths, but all shorter than 

 the sixth-metret. The shortest are about four and the longest about 

 eight of the seventh-metrets, but none so long as ten seventh-metrets, 

 which would make up the whole of a sixth-metret. They must there- 

 fore take rank with microscopic objects so small that they can only 

 be seen with a tolerably high power. Small as they are, these tiny 

 waves advance with extraordinary speed, travelling a distance of 

 thirty quadrants of the earth in a second of time, meaning by a 

 quadrant the distance along a meridian from the earth's equator to 

 the pole, a distance which measures ten millions of metres. The 

 vibratory motion at each point of space is transverse to the direction 

 in which the waves are travelling, a kind of motion with which we 

 are familiar in the waves that run along the surface of water. The 

 range of this transverse motion is about a tenth-metret, the very 

 minute quantity to which I drew attention a while ago; and the 

 rapidity with which it is repeated varies with the colour of the light, 

 and for green light has about its mean value : in a ray which pro- 

 duces this colour the oscillatory movement is repeated about as often 

 every second as there are seconds in nineteen millions of years. 



This short sketch will, I hope, give a picture that will suffice for 

 our present purpose, of the kind of motion in the outer world that 

 affects us through our organ of sight. It stands in very broad 

 contrast to that great coarse kind of motion which acts upon us 

 through our ears, and which for the middle E of the piano is repeated 

 only 330 times in a second. The direction of motion also is different, 

 for in the sound waves of air it is backwards and forwards along the 



