OBSER VA TIOX. 



It is now apparent that what we call light is the affection 

 of the eye by certain vibrations, the less rapid of which 

 are invisible and constitute the dark rays of radiant heat, 

 in detecting which we must substitute the thermometer or 



o 



the thermopile for the eye. At the other end of the 

 spectrum, again, the ultra-violet rays are invisible, and 

 only indirectly brought to our knowledge in the pheno- 

 mena of fluorescence or photo-chemical action. There is 

 no reason to believe that at either end of the spectrum an 

 absolute limit has yet been reached. 



Just as our knowledge of the stellar universe is limited 

 by the power of the telescope and other conditions, so our 

 knowledge of the minute world has its limit in the powers 

 and optical conditions of the microscope. There was a 

 time when it would have been a reasonable induction that 

 vegetables were motionless, and animals alone endowed 

 with power of locomotion. We are astonished to dis- 

 cover by the microscope that minute plants are if any- 

 thing more active than minute animals. We even find 

 that mineral substances seem to lose their inactive 

 character and dance about with incessant motion when 

 reduced to sufficiently minute particles, at least when sus- 

 pended in a non-conducting medium f . Microscopists will 

 meet a natural limit to their means of observation when the 

 minuteness of the objects examined becomes comparable 

 to the length of light undulations, and the extreme diffi- 

 culty already encountered in determining the forms of 

 minute marks on Diatoms appears to be due to this cause. 



Of the errors likely to arise in estimating quantities by 

 the senses I have already spoken (vol. i. p. 320), but there 

 are some cases in which we actually see things different 

 from what they are. A jet of water often appears to be 

 a continuous thread, when it is really a wonderfully or- 



f Jevons, ' Proceedings of the Literary and Philosophical Society of 

 Manchester,' 2,5th January. 1870, vol. ix. p. 78. 



