36 



groups, ruled to different scales, some have 30 bauds or 

 groups, the coarsest having its lines 1-lOOOth of a Paris line 

 apart, and the finest being l-8000th — each group or band 

 being about l-2000th of an English inch in width, and the 

 whole 30 occupying a space perhaps a little more than l-50th 

 of an inch. 



The plates now most in use have 19 bands, the first or 

 coarsest being ruled to 1-11, 240th of an English inch, and the 

 finest to 1-11 2,668th of an inch. It is a difficult matter for 

 the mind to appreciate such minute divisions, yet it is essential 

 it should conceive them to prepare it for estimating the diffi- 

 culty of seeing and counting such lines. 



Is it possible for human art ever to make an instrument 

 capable of rendering lines 112,668 to an inch visible. Is there 

 anything in the laws of light which renders it impossible to 

 see lines so close, and is it therefore uselesss for the optician 

 to attempt to improve the microscope beyond a certain point? 

 Or, for the naturalist to try any further investigations on the 

 structure of tissues beyond what the present existing instru- 

 ments have shown. This problem is the exact parallel of that 

 in testing the quality of the telescope for separating double 

 stars. In 1855, Professor Queckett, asserted that " no achro- 

 matic has yet been made capable of separating lines closer 

 together than l-75,000th of an inch." " Mr. Eoss found it 

 impossible to ascertain the position of lines nearer than 

 l-80,000th of an inch." " Mr. De la Eue was unable to 

 resolve any lines on Nobert's test-plate, closer than l-81,000th 

 of an inch." "Dr. W. B. Carpenter says, that no objective 

 will, probably, ever be able distinctly to resolve lines closer 

 than l-84,000th of an inch." If these observations are cor- 

 rect, the existence of lines finer than this is a matter of faith 

 rather than sight ; there can be no reasonable doubt, however, 

 that lines do exist, and the resolution of them would evince 

 the extraordinary superiority of any objective, or any system 

 of illumination, which would enable them to be distin- 

 guished." 



The late Professor J. "W. Baily, claimed to have seen lines 

 as close together as l-100,000th of an inch, and Messrs. Har- 

 rison and Sollitt of Hull, claimed to have measured lines on 

 the Diatom Ampld-pJem^a-pelliicida, as fine as 120,000, to 

 130,000 to the inch. E. C. Greenleaf of Boston, and Mr. 

 Charles Stoddard, were well satisfied that they saw the lines 

 90,000 to the inch, with a Tolles l-5th, and afterwards Mr. 

 Greenleafe saw the same lines very distinctly with a Tolles 

 l-12th. Dr. J. J. Woodward, of Washington, states that with 

 mono- chromatic light, and Powell and Lealand's l-50th, and 

 l-25th, and l-16th objectives a Hartnach's immersion No. 11, 



