WHAT THIS WORLD IS MADE OF 



will be found full of threads so delicate as to be 

 scarcely visible under high magnifying powers. 



The limits of visibility with the microscope have 

 been very carefully measured, and are from yg-ffJoTrfr 

 to the yoisWff of an inch, about. So that the chro- 

 matic threads in the protoplasm are somewhat less 

 than a hundred-thousandth part of an inch thick. 

 This is a very clumsy way to express it ; it is like 

 saying that a lead-pencil, for example, is about a 

 hundred and fifty thousandth of a mile in diam- 

 eter; so more convenient units have been chosen. 

 The shortest wave-lengths of light might be em- 

 ployed as a standard; this is about one sixty- 

 thousandth of an inch. 1 The chromatic threads 

 of protoplasm, then, would be somewhere around 

 half a wave-length thick. 



But for scientific measurements the metric sys- 

 tem is in universal use, and the wave-lengths of 

 light happen to be fractional parts of any decimal 

 division of a metre. So the even millionth of a 

 metre has been agreed upon for microscopic meas- 

 uring. It is called a micron, from the Greek word 

 for small, and is written with the Greek letter /i. 

 It is about a millionth of a yard, or the twenty -five 

 thousandth of an inch. For still smaller spaces, as 

 we have seen, this is again divided by one thou- 



1 See "The World Beyond Our Senses," p. 41. 

 105 



