Introduction to Hering's Lecture 59 



" Who would not," ^ says Sir John Herschel, " ask for 

 demonstration when told that a gnat's wing, in its ordinary 

 flight, beats many hundred times in a second ? or that 

 there exist animated and regularly organised beings many 

 thousands of whose bodies laid close together would not 

 extend to an inch ? But what are these to the astonishing 

 truths which modern optical inquiries have disclosed, 

 which teach us that every point of a medium through 

 which a ray of light passes is affected with a succession 

 of periodical movements, recurring regularly at equal 

 intervals, no less than five hundred millions of millions of 

 times in a second ; that it is by such movements com- 

 municated to the nerves of our eyes that we see ; nay, 

 more, that it is the difference in the frequency of their 

 recurrence which affects us with the sense of the diversity 

 of colour ; that, for instance, in acquiring the sensation of 

 redness, our eyes are affected four hundred and eighty-two 

 millions of millions of times ; of yellowness, five hundred 

 and forty-two millions of millions of times ; and of violet, 

 seven hundred and seven millions of millions of times 

 per second ? ^ Do not such things sound more like the 

 ravings of madmen than the sober conclusions of people 

 in their waking senses ? They are, nevertheless, con- 

 clusions to which any one may most certainly arrive who 

 will only be at the pains of examining the chain of reasoning 

 by which they have been obtained." 



A man counting as hard as he can repeat numbers one 

 after another, and never counting more than a hundred, 

 so that he shall have no long words to repeat, may perhaps 

 count ten thousand, or a hundred a hundred times over, 

 in an hour. At this rate, counting night and day, and 

 allowing no time for rest or refreshment, he would count 

 one million in four days and four hours, or say four days 



1 Discourse on the Study of Natural Philosophy. Lardner's 

 Cab. Cycle, vol. xcix. p. 24. 



" Young's Lectures on Natural Philosophy, ii. 627. See also Phil. 

 Trans., 1801-2. 



