28 CORRESPONDENCE. 



magnifyiug power of my Powell and Lealaud's ^\tli when used with 

 its immersion front, so that it now only remains to say a word on the 

 difference which Mr. Stodder refers to between his Xobert's plate and 

 mine. Mr. Tolles had Mr. Stodder's plate with him during his visit ; 

 it is moimted on a thick bottom glass, and is a facsimile of one be- 

 longing to Dr. Barnard, which served for my early work on the plate. 

 I was therefore perfectly familiar with its characters. On the other 

 hand, the plate now^ used in the Museum is mounted on a very thin 

 bottom glass, permitting therefore the use of a condenser of short 

 focus and high angle. The chief points of difference between the 

 two forms of plate are given in a printed ' Description of the Nine- 

 teen Band Test-plate of Nobert,' issued nearly two years ago from the 

 Museum, with a series of photogi'aphs of the lines, and a copy of 

 which is in Mr. Stodder's possession. I am at a loss therefore to 

 understand his remarks in this connection, but may add to what I 

 have heretofore wi'itten, that on thick-bottomed plates, like Mr. 

 Stodder's, the spurious lines (counting from thii-ty to forty iu the 

 nineteenth band, for example) are even more decej^tively like the true 

 lines than on thin-bottomed plates, and that a count is even more 

 indispensable in the first instance than in the second. 



I must also ask of you space for a few words with regard to two 

 communications in jo\w November niimber. On jjage 241 a corre- 

 spondent, who signs himself B., and who seems to wi-ite very fairly, 

 asks of me the apertui'e of the Tolles's ith mentioned in my Note on 

 Amjjhijjleura. I take pleasure in giving the information desired. The 

 lens in question has two fronts — one of moderate angle for use dry. 

 I make its aperture 90" at uncovered, 110° when corrected for cover 

 by one full turn. The other front may be used either dry or wet. 

 When the posterior combination is separated from the front as far as 

 the screw collar will move it, the objective is fitted for the study of 

 dry uncovered objects, and has an angle of 110^. One full turn of 

 the screw collar corrects the objective for the thickest cover through 

 which it will work di-y. Its angle is then 140". Without further 

 change of adjustment a drop of water may be introduced, and it is 

 fitted for the study of uncovered objects wet ; anotlier turn and a half 

 may then be given, when it is corrected for the tliickest cover through 

 which it will work wet, and has an angle of 170° or upwards. The 

 price paid Mr. Stodder for this objective, including both fronts, was 

 one hundi-ed dollars (United States' currency). 



In yoiir November number, also, on page 225, is a paper by Mr. 

 Edwin Bicknell, of Massachusetts, who wilfully misinterprets my pub- 

 lished measui'ements of the wet front of Powell and Lealand's yV*^' 

 and thinks proper to use the following words (p. 226) : — " I call it 

 nothing more nor less than deception in Powell and Lealand (or any 

 other makers) in marking (sic) an objective nearly 33 per cent, less 

 than its actual power, thus misleading people who cannot make 

 actual comparisons ; and I consider Dr. Woodward guilty of an equal 

 amount of deception in knowingly putting forth work done by that 

 objective as having been done by a jV*^-" T^^^^ language is vulgar, 

 insolent, and offensive, and I should take no further notice of the paper 

 in which it occurs if it did not seem to me that fair play to Messrs. 



