a7id its Observers. 121 



admit it, because he did not count all the lines. As Dr. Hagen has 

 been replied to,* the reply to him will answer also for Dr. W. The 

 position assumed by both gentlemen is absurd ; for " suppose that 

 Nobert had ruled in the 19th band only twenty-eight lines instead 

 of fifty-seven, would Dr. H. say that they were not resolved because 

 there were no more ? Or if Nobert had covered a whole inch with 

 the 112,000 and some odd lines, would anyone claim that all must 

 be seen at once ? If either of these suggestions is answered in the 

 negative, then Dr. Hagen has himself seen the 19 th band resolved 

 with a Tolles' objective." 



The evidence of the resolution of the lines is in the progressively 

 increasing fineness of each band, as each is successively resolved ; 

 and in the experience of the observer, in distinguishing the differ- 

 ence between difii-action, spectral, or spurious, and the true lines. 



I will now introduce my witnesses, who are tolerably well known 

 among microscopists, so that I hope Dr. Woodward will admit their 

 " competency." I am forced as it were to this course by Dr. Wood- 

 ward's persistence in advertising that he does " not believe " me. 



First, a letter from Mr. Edwin Bicknell, of the Zoological 



Museum, Cambridge. 



Cambridge, Sejjt. 8, 1870. 



Deab Sir, — I desire to state that in August, 1868, 1 called at Mr. 

 Tolles' room in Hanover Street, and while there had an opportunity 

 of viewing the test-plate of Nobert. I distinctly saw the 19th baud 

 (if I can trust my eyes) well resolved, the lines sharp and distinct ; 

 with no special lines, and each line of the band much narrower than 

 each space between the lines. While I regard counting a positive 

 proof of resolution (that is, true lines, not sjjectral lines, which can 

 be counted as well as jihotographed), I do not think it absolutely 

 necessary that we should doubt our eyesight until a count is made 

 and compared with somebody else's count or measurement. In case 

 an actual count is demanded as proof of resolution, we shall have to 

 throw away all our supposed resolutions oiNav. rJiomhoides, Siir. gemma, 

 PL fasciola, and many other common test-objects, and go to the trouble 

 of verifying by count and measurement before we can trust our work. 

 Observers who can be considered " experts " on the subject, differ as 

 to whether a count is actually needed, or is not needed, as proof of 

 resolution ; it is a matter of individual judgment, and rules cannot 

 well be enforced. At the time of my seeing the 19th band of the 

 test-plate, I had no difficulty in counting a portion of the band, but 

 did not undertake to count the whole of it, as it would have been very 

 difficult, there being no resting-jjoints for the eye ; and the whole 

 band would have had to be counted without stopj)ing, which with the 

 low power used (^th immersion) would have been impossible for me 

 to have done. I am, very truly yours, 



Edwin Bicknell. 



* 'American Naturalist' for Sept., 1870, p. 425 ; and 'Monthly Microscopical 

 Journal,' November, 1870. 



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