COERESPONDENCE. 203 



between two, tlirce, five, or ten lines of a certain band of the plate he 

 has resolved it. If he doubts the ruling, or wishes to verify he 

 may count and measure until he is satisfied. It was entii-ely optional 

 with the artist to rule ten lines or ten hundred — the j)roblem of reso- 

 lution is the same. There is no special virtue in fifty-seven lines. Dr. 

 Woodward's criterion of the resolution is as much a test of the " flat- 

 ness" of the glass on which the lines are ruled, as of the " flatness" of 

 the field of the objective. It will readily be perceived by anyone who 

 has tried the resolution, that so minute and delicate are the lines, that 

 any curvatui'e of the glass, or variation even in the thickness, will throw 

 some of the lines out of focus. 



I readily acknowledge the justness of one of Dr. W.'s criticisms 

 on my original paper. It was an oversight to suggest that the micro- 

 meter must be moved the 100,000th of an inch only. That, however, 

 does not remove the difficulty of counting " such fine lines." I never 

 said it was impossible. 



I regret exceedingly that Dr. W. should think that I have shown 

 unfairness to him in representing his remarks : it was my endeavour to 

 represent them fairly, and I believe I have represented them as they 

 were understood by all my friends. I was acquainted with Dr. 

 Hagen's assertion which Dr. W. quotes, though I do not read the ori- 

 ginal I know that Dr. Hagen wrote that none of Tolles' objectives had 

 resolved the nineteenth band, and wrote it after he had been positively 

 told by several accomplished microscopists that they had seen it re- 

 solved; equivalent to saying that they did not know what they had 

 seen so well as Dr. H. did himself ; and since that time Dr. Hagen has 

 said that he had seen the true lines of the nineteenth band with a 

 Tolles' y^i^. It of course is a question what constitutes a resolution, 

 but he did not suggest that. I have defined my problem, and submit 

 it to the microscoijical world for their approval or rejection. 



It is a misfortune that so much time elapses between the making 

 observations, and writing and the publication. My inquiry of Dr. W. 

 was written in September, 1870, though it was from causes beyond my 

 control in my hands until November, and it was not published until 

 March, My question referred of course to the time (May, 1869) when 

 Dr. W. tried the Tolles' objectives. He replies (April ?) 1871, stating 

 what the objectives will do that he has then. If he had delayed until 

 July, a change had occui-red. He has now ascertained that the Powell 

 and Lealand so-called |th objectives are really yV^h ; the so-called 

 ^\th is a -pjfth ;* that his description of the appearance of the true 

 lines of the nineteenth band on Ms plate is not a correct descrij^tion 

 of the appearance of lines of the same band on the plate I used. All 

 these facts, I presume. Dr. W. will in due time communicate to the 

 public. 



In conclusion. Dr. Woodward is entitled to the thanks of all 

 microscopists, who may undertake the excessively difficult task of re- 

 solving these lines, for the admirable manner in which he has described 



* Objectives are named when adjusted for uncovered objects, a fact not 

 generally known by purchasers. The power increases, i. e. the focus is shorter 

 as the collar is turned to work through the covering glass. 



