366 HOW TO SEE WITH THE MICROSCOPE. 



It was my intention to pass over one or two points presented 

 by Prof. Hitchcock, but fearing that silence may be miscon- 

 strued, a word or two may not be amiss. Prof. Hitchcock does 

 not deny that Dr. Carpenter has lately (note that point) given 

 his unqualified endorsement to the superiority of the duplex 

 glasses, but he adds, " so does everybody else." This, too, must 

 be another slip of the pen, for it is all, yea, more than I have 

 claimed, for at the date of my Dunkirk lecture, the claims of 

 the duplex were not admitted by everybody. During the past 

 four years I have, myself, received hundreds of letters from as 

 many sceptics. 



Prof. Hitchcock states that he was taken "a little by surprise" 

 by my claim that the " very best work of the duplex is seen by 

 central light," and with singular tact allows the little fact to 

 remain intact. This is too bad, for right here I expected the 

 thickest of the fight. It was, in fact, " my chosen ground." 

 Prof. H. is too much for me on tactics. 



In this connection, I beg the reader to bear in mind that 

 while Prof. II. admitted the superiority of wide angles for the 

 resolution of " diatoms and Robert's test-plates," he neverthe- 

 less held that this did not support my view that high angles are 

 " universally preferable," (see this journal, May, 1877). To 

 meet this point, I stated in my response (this journal, August, 

 1877,) that I had not omitted such tests as are generally consid- 

 ered " chosen ground " for small apertures, i. e., what is gener- 

 ally understood and accepted as dead central illumination ; and 

 further, that among other tests I selected Navicula angulata 

 illuminated by central light, because such resolution had been 

 publicly declared impossible. In reply Prof. II. kindly admits, 

 and in complimentary terms, all that I had claimed as to cen- 

 tral light work, and he frankly adds, " they are just what we 

 would naturally expect a priori." Having made this full admis- 

 sion, Prof. H. seems to repent a little, and reverts to the sub- 

 ject again (page 112), still yielding that the high angles are 

 superior, both with oblique or central illumination, but for reso- 

 lutions only, because he says, the low angles have the greater 

 penetration. The nature of this, his residual error, is mani- 

 festly indicated in the piesent paper. Less than IV Uvo hours 



