AND MODERN MICROSCOPES. 133 



renders useless the labours of the optician to improve his in- 

 struments beyond a certain point ? and, as a corollary, is it 

 decided that it will be useless for the naturalist to try to 

 investigate the structure of tissues beyond what the best 

 existing instruments have shown ? It must be borne in mind 

 that the power of seeing a single object is not the question, 

 but the power of distinguishing two or more objects nearly 

 in contact. The problem is exactly the parallel of that of 

 the power of the telescope of separating double stars. A 

 brief sketch of what has been done and what opinions on the 

 problem have been expressed by eminent microscopists and 

 opticians is essential to a full understanding of the question. 



Professor Quecket, in 1855, asserted that "no achromatic 

 has yet been made capable of separating lines closer together 

 than the Y-rhnj ^^ ^^^ inch." " Mr. Hoss found it impossible 

 to ascertain the position of a line nearer than -g ^ ^ ^ q. of an 

 inch." " Mr. De la Rue was unable to resolve any lines on 

 Nobert's test-plate closer than -«t-s^-o of an inch." 



Dr. William B. Carpenter, in his work on the micro- 

 scope, published in 1856, says, " Even the Vt objective will 

 probably not enable any band to be distinctly resolved 

 whose lines are closer than . g ^ „ ^ of an inch. At present, 

 therefore, the existence of lines finer than this is a matter of 

 faith rather than of sight ; but there can be no reasonable 

 doubt that the lines do exist, and the resolution of them 

 would evince the extraordinary suj^eriority of any objective, 

 or of any system of illumination, which should enable them 

 to be distinguished." In his second edition, issued in 1859, 

 Dr. Carpenter repeated the same remarks, but substituted 

 8-3 j, for 7-9-5-0-0? and then added, " There is good reason to 

 believe that the limit of j)erfection (in the objective) has now 

 been nearly reached, since everything which seems theoreti- 

 cally possible has been actually accomplished." In the third 

 edition, 1862, he again alters the figures to -5-4^0-0-^ but adds 

 nothing more. 



On the other side the late Professor J. W. Bailey claimed 

 to have seen lines as close together as , o'o to the inch, 

 and Messrs. Harrison and Solitt, of Hull, England, claimed 

 to have measured lines on the diatom Amphipleura pellucida 

 as fine as 120,000 to 130,000 to the inch, and expressed the 

 opinion that lines as fine as 175,000 might be seen with 

 proper means. 



To determine, if possible, the truth between these confiict- 

 ing opinions, Messrs. Sullivaift and Wormly (' American 

 Journal of Science,' January, 1861) made an exhaustive trial 

 of one of these '^ marvels of art." They state that the opti- 



