MICROSCOPY. 



By Professor HAMILTON L. SMITH, 



Hobart College, Geneva, N. Y. 



LIMITS OF VISION. 



Reasoning on certain data more or less theoretical, raathe- 

 maticians of the first order, notably Helraholtz, had conclud- 

 ed that the limit of vision had been reached ; that the opti- 

 cian could practically aid ns no further; that, in short, the 

 limits of possibility had been arrived at, since light itself is 

 too coarse to reveal objects smaller than those visible to our 

 finest and most powerful lenses. The limit marked out was 

 about the one -hundred -and -eighty- thousandth of an inch. 

 Recently the Rev. Mr. Dallinger, in a note read before the 

 Liverpool Microscopical Society, gave instances of a remark- 

 able kind the result of his personal investigation, directed 

 specially to this point which were proved, by a method of 

 measurement employed specially for the purpose, to carry 

 the power of our most delicately constructed lenses consid- 

 erably further than the mathematician considered possible, 

 revealing, indeed, smaller objects than those mathematically 

 indicated; and Mr. Dallinger did not by any means believe 

 that he had wholly exhausted the power of visibility by these 

 experiments. In reference to the same subject Mr. W.Webb 

 states, in the Monthly Microscopical Journal for October, 

 that Mr. Crisp, of London, has in his possession a diamond 

 engraving of the Lord's Prayer in which the letters are small- 

 er than the two-hundred-and-ten-millionth part of a square 

 inch, at which size over fifty-nine Bibles would be required 

 to cover an inch. Mr. Webb criticises somewhat severely 

 the paper of Mr. Rogers, of Cambridge, Mass., " On a Possi- 

 ble Explanation of the Method employed by Robert in Rul- 

 ing his Test Plates," and makes some very obscure state- 

 ments, to the effect that the spurious lines are caused by 

 polarization of the light ! According to the results of the 

 undulatory theory of light, the size of the fringes of diffrac- 



