How few people have ever realised what a wonderful Counting Machine they possess in their organ of Sight! I think the best method I can adopt, to bring this clearly before you, is to take our tuning-fork, vibrating 500 times per second, a rapidity which to some will be even difficult to comprehend, and then ask you to consider how long that fork must continue to vibrate before it has accomplished the full number of frequencies, which must necessarily impinge upon the eye in one second of time, before the phenomenon of sight becomes possible. That tuning-fork would have not only to continue its vibrations without diminution for seconds, minutes, hours, weeks, months, years, or hundreds of years, but for 30,000 years before it has accomplished the full number of pulsations which, as Ether waves, must strike the eye in one second of time, to give the impression of Light; the calculation is easy, the rills of Red Light are so small that 40,000 of these only cover one inch of length, and light travels 186,000 miles per second. If therefore the number of inches in 186,000 miles are multiplied by the 40,000, and the product is divided by the 500 times which the tuning-fork vibrates in one second, you have the number of seconds that tuning-fork must vibrate, before it has completed the number of impacts which, in one second of time, must fall on our retina to give us the impression of red light; and that tuning-fork would have to vibrate nearly twice as long, say 50,000 years, to reach the number of impulses which strike the eye in one second of time and give the impression of violet light; and between these two limits are situated the colours--Orange, Yellow, Green, Blue, and Indigo.