PHILOSOPHICAL SOCIETY OF WASHINGTON. 155 



to that principle we must in seeking for the simplest idea of physi- 

 cal force have regard to the sense of touch."* Let us inquire then 

 what is taught us by tactile experience with regard to the philoso- 

 phy of physical contact. In the celebrated experiment by which 

 Newton first measured the wave-lengths of light from the colored 

 rings which yet bear his name, he found that on placing a piece of 

 clean plate glass upon the convex surface of a large lens, a very 

 considerable pressure was required to exhaust the series of outcom- 

 ing interference fringes and to exhibit the central black spot. Pro- 

 fessor Robison estimated that a pressure of at least one thousand 

 pounds to the square inch was necessary to effect this approach to 

 a mathematical contact between the two glasses.f And yet even 

 with this very close and perfect physical contact it is shown that at 

 the first appearance of the black spot between the glasses, they are 

 still separated from actual or mathematical contact by the space of 

 the 250,000th of an inch. 



Material Contact not Absolute. — Supposing it were desired to di- 

 rectly communicate a push or a pull through the distance of seven 

 miles, a perfectly straight steel bar (properly supported on friction 

 rollers through that space) would probably be as efficient a mechan- 

 ical means for the purpose as could well be suggested. And yet the 

 blow of a suitably heavy hammer struck upon one of its ends would 



* Prof. James Challis. Principles of Mathematics and Physics. 1869 : 

 p. 358. 



-\A System of Mechanical Philosophy. By Prof. John Kobison : vol. I, 

 sect. 241, p. 250. Dr. Young remarks on this: "Hence it is obvious that 

 whenever two pieces of glass strike each other without exerting a pressure 

 equal to a thousand pounds on a square inch, they may effect each other's 

 motion without actually coming into contact. Some persons might per- 

 haps be disposed to attribute this repulsion to the elasticity of particles of 

 air adhering to the glass, but I have found that the experiment succeeds 

 equally well in the vacuum of an air-pump. We must therefore be con- 

 tented to acknowledge our total ignorance of the intimate nature of forces 

 of every kind." (Lectures on Natural Philosophy. 2 vols. 4to. London, 

 1807: lect. Ill: vol. i, p. 28.) And Prof. J. Clerk Maxwell says to the 

 same effect: "We have no evidence that real contact ever takes place be- 

 tween two bodies, and in fact when bodies are pressed against each other 

 and in apparent contact, we may sometimes actually measure the distance 

 between them, as when one piece of glass is laid on another, in which case 

 a considerable pressure must be applied to bring the surfaces near enough 



