xiii.] MEASUREMENT OF PHENOMENA. 297 



way in methods of minute measurement. He did not 

 call waves of light by their right name, and did not 

 understand their nature ; yet he measured their length, 

 though it did not exceed the 2,ooo,oooth part of a metre 

 or the one fifty-thousandth part of an inch. He pressed 

 together two lenses of large but known radii. It was 

 easy to calculate the interval between the lenses at any 

 point, by measuring the distance from the central point 

 of contact. Now, with homogeneous rays the successive 

 rings of light and darkness mark the points at which the 

 interval between the lenses is equal to one half, or any 

 multiple of half a vibration of- the light, so that the 

 length of the vibration became known. In a similar 

 manner many phenomena of interference of rays of light 

 admit of the measurement of the wave lengths. Fringes 

 of interference arise from rays of light which cross each 

 other at a small angle, and an excessively minute dif- 

 ference in the lengths of the waves makes a very perceptible 

 difference in the position of the point at which two rays 

 will interfere and produce darkness. 



Fizeau has recently employed Newton's rings to measure 

 small amounts of motion. By merely counting the number 

 of rings of sodium monochromatic light passing a certain 

 point where two glass plates are in close proximity, he is 

 able to ascertain with the greatest accuracy and ease the 

 change of distance between these glasses, produced, for 

 instance, by the expansion of a metallic bar, connected with 

 one of the glass plates. 1 



Nothing excites more admiration than the mode in which 

 scientific observers can occasionally measure quantities, 

 which seem beyond the bounds of human observation. 

 We know the average depth of the Pacific Ocean to be 

 14,190 feet, not by actual sounding, which would be 

 impracticable in sufficient detail, but by noticing the 

 rate of transmission of earthquake waves from the South 

 American to the opposite coasts, the rate of movement 

 being connected by theory with the depth of the water. 2 

 In the same way the average depth of the Atlantic Ocean 

 is inferred to be no less than 22,157 f eet > from the velocity 



1 Proceedings of the Royal Society, 3oth November, 1866. 



2 Herschel, Physical Geography, 40. 



