136 STODDER, ON NOBERT's TEST-PLATE 



With Tolles' -^ immersion, angular aperture 170°, B eye- 

 piece, power 550, Mr. Greenleaf and myself both saw the 

 19th band satisfactorily. Thus being probably the first ever 

 to see lines of 112,000 to the inch, and establishing the fact 

 of the visibility of such lines, contrary to the theory of the 

 physicists. (It should, however, have been mentioned in 

 the proper place that Mr. Eulenstien says that Nachet claims 

 to have seen them by sunlight recently, which claim needs 

 some confirmation, as his No. 10 failed so completely in my 

 hands.) 



In the present month (January, 1868), Dr. F. A, P. 

 Barnard writes to Mr. Greenleaf, that he had tried several 

 objectives, naming a Spencer -t?j^ and -V, a Tolles' -j\ and -i-, 

 a Wales ^, and a Nachet immersion No. 8, equal to a ^-V. 

 " The Spencer -jL and the Nachet -jV broke down at about 

 the 11th or 12th band. With the Wales J- I got as far as 

 ten, or perhaps eleven bands. With the Tolles' i I made 

 out distinctly ten." 



In another communication he says, " The highest band I 

 can count is the 16th." In a more recent letter to the Avriter 

 Dr. Barnard gives the count of the lines on a portion of his 

 plate, corresponding as nearly as could be expected to 

 figures given in the table up to the 14th; but the 16th band 

 he could not count satisfactorily, different attempts giving 

 varying results. It has been said that the resolution of the 

 lines to the eye implies the ability to count them, but this I 

 think is a fallacy ; a few lines of a grouj) may be counted 

 correctly, and then it becomes difficult to identify the line 

 last counted and the one to be counted next. Let any one 

 try to count the pickets in a fence, Avhen the pickets are 

 distinctly visible, say at a distance of 100 or 150 yards, he 

 will find this difficulty almost insurmountable. In the micro- 

 scope the micrometer is an aid in counting, but in counting 

 lines of such exquisite fineness either the micrometer or the 

 stage must be moved, and it is next to impossible to construct 

 apparatus that can be moved at once towtt-o of an inch and 

 no more. It would require the genius and skill of Nobert 

 himself to do it. 



These trials show conclusively that it is not the great 



Tolles' immersion ^l, anj^. ap. about 1C0°, B eye-piece, 



power about 800, sunliglit central . . . 8tli band, 



Tolles' immersion -^-^, anp. ap. about 160°, B eye-piece, 



power about 800, sunlight oblique . . . 12th „ 



Tolles' imiTiersion J^, ang. ap. about 160°, B eye-piece, 



petroleum, light oblique .... 12th „ 



Tolles' immersion ■j'5> o'^ another occasion I saw the . 15th „ 



