406 THE PRINCIPLES OF SCIENCE. [CHAK 



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 are 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. 1 Microscopists will 

 meet a natural limit to observation when the minuteness 

 of the objects examined becomes comparable to the length 

 of light undulations, and the extreme difficulty already 

 encountered in determining the forms of minute marks on 

 Diatoms appears to be due to this cause. According to 

 Helmholtz the smallest distance which can be accurately 

 defined depends upon the interference of light passing 

 through the centres of the bright spaces. With a the- 

 oretically perfect microscope and a dry lense the smallest 

 visible object would not be less than one 8o,oooth part 

 of an inch in red light. 



Of the errors likely to arise in estimating quantities by 

 the senses I have already spoken, but there are some cases 

 in which we actually see things differently from what 

 they are. A jet of water appears to be a continuous 

 thread, when it is really a wonderfully organised succes- 

 sion of small and large drops, oscillating in form. The 

 drops fall so rapidly that their impressions upon the eye 

 run into each other, and in order to see the separate drops 

 we require some device for giving an instantaneous view. 



One insuperable limit to our powers of observation 

 arises from the impossibility of following and identifying 

 the ultimate atoms of matter. One atom of oxygen is 

 probably undistinguishable from another atom; only by 



1 This curious phenomenon, which I propose to call pcdesis, or the pedctic 

 movement, from -miSo'ta, to jump, is carefully described in my paper published 

 in the Quarterly Journal of Science for April, 1878, vol. viii. (N.S.) 

 p. 167. See also Proceedings of the Literary and Philosophical Society 

 of Manchester, 25th January, 1870, vol. ix. p. 78, Nature, 22nd August, 

 1878, vol. xviii. p. 440, or the Quarterly Journal of Science, vol. viii. 

 (N.S.)p. 514. 



