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‘been produced I have obtained them each and all, that had any real value, 
‘whether produced in this country, the continent, or America; and, in some 
‘‘cases, I have incited certain English makers to produce certain special 
‘‘formula during that time. But while I have used all lenses, from the 4th 
‘to the 1-5oth, constantly during this time, what work I have done could 
‘never have been accomplished, if I had only had lenses with /arge angles to 
‘‘ work with. Much that had been done could never have been done without 
‘« them ; but the work, as a whole, could never have been done at all if only 
‘such had been at my disposal. Hence I have, in all my special working 
** powers, ¢hree lenses of the same power, and, in some cases, four, and each of 
‘them in following out the details of a life-history of an organism, say of the 
‘*1-3000th to the 1-6000th of an inch in length is absolutely needed, and its 
an cannot be supplied by the other. Thus, I have two 1-50ths, one 
‘having a very low angle ”—[fancy a low angle 1-50th; what will he call a low 
angle—1-50?]—“‘and the other as great a numerical angle as an oil immer- 
‘sion can provide when worked by the best makers. In the 1-35th, I have 
‘* but one lens, a medium angle, because it was intended only for general work 
‘‘and, mainly, central illumination. I have, however, three 1-25ths, four 
‘¢1-16ths, and so on; and I know exactly what each will do, and no more 
‘‘ attempt to get the work of one out of the other than the maker of them would 
‘‘attempt to get their several results by grinding them to the same formulz. 
‘*(3) I talked this matter over in detail, pointing out results six years ago with 
‘some leading experts; and although, during two or three years, many have 
‘*thought that Abbé’s mathematics and views were adverse to this view of 
‘« mine, I felt convinced by reading between the lines of his papers, and remem- 
‘* bering their special object, that it was not so. Still, Dr. Carpenter was good 
‘enough to get a detailed view of my experience and opinion before publishing 
“the last edition of Zhe Microscope, and he has in his preface and throughout 
‘the volume, given in effect my views, which now the unmistakable declara- 
‘tions of Abbé coincide with and confirm. (4.) The homogeneous lenses have 
‘« given me splendid results, some of which will shortly be published ; but xo 
‘« immersion lens of any kind couLD be used to work out to the end an organic 
“‘life-history—that is, if it involved life and movement, because the object 
** being in a limited area, and (possibly) in fluid, the fluid under the cover does, 
‘* by following the movements of the object, at length, without the spectator’s 
‘knowledge, mingle with the fluid above (employed for the lens) and thus 
‘* destroy the whole object of search and study. I would have given a diagram 
«© of this as it is most important, but I hope you will understand, for I have no 
‘further time. This fact, then, makes air angles of the highest importance, 
‘and I hope the highest results have not yet been attained with them. In the 
main, then, I agree with Abbé. I agreed with his former papers, with the 
‘reservation only which is now no longer needful, because his last paper, to 
‘«which you refer, expresses and admits what my reservation implied. This 
“letter is written in haste, and without an opportunity for revision. 
‘* Believe me, truly yours, ‘‘W. H. DALLINGER.” 
In addition to that I wrote to Professors Huxley and Tyndall. I have re- 
ceived a note from Tyndall. It appears my letter followed him all the way to 
Switzerland. It is dated the 28th August. Now, this letter is very short, but 
it certainly carries out one idea which I agree with, and that is that, after all, 
ou could carry out good scientific microscopical work without bothering your 
eads about angles at all, (Cheers.) The letter is as follows :— 
«“ ALp LUSGEN, BRIEG, SWITZERLAND. 
“ DEAR S1R,—I should willingly aid you in your inquiries, but my knowledge 
‘* would not warrant any expression of opinion on my part as to ‘the aperture 
‘* question in connection with microscopical investigation.’ In my own obser- 
«‘ vations with microscopes, I have simply accepted the means which the skill 
«¢ of others placed at my disposal. “Yours truly, 
** August 28th, 1882,” “JOHN TYNDALL. 
