Y8 



THE AMERICAN MONTHLY 



[April, 



course in this regard, and we are 

 pleased to observe that the products 

 of his workshop are now steadily 

 growing in favor. We refer to Mr. 

 William Wales. We do not mean to 

 imply that the course followed by the 

 manufacturers has been productive of 

 no good. Undoubtedly it has resulted 

 in much valuable experience, which 

 will sometime be shown in a better 

 class of objectives. But with this 

 matter we have nothing to do at 

 present. 



What we desire to impress upon the 

 minds of our readers is the fact that 

 resolving power alone is not a test to 

 be depended upon. The reason for 

 this will be obvious to anyone who 

 has studied the articles already pub- 

 lished in this Journal. When a dif- 

 ficult test is resolved, the extremely 

 oblique rays that enter the objective 

 are the only ones that contribute to 

 the formation of the image of the 

 lines. All the central portion of the 

 objective might as well be stopped 

 out. An annular zone around the 

 periphery takes in all the light that is 

 necessary to form the image. It is, 

 therefore, a simple matter for the 

 maker to correct the objective, 

 especially for the rays entering that 

 annular zone, the result of which 

 would be, the objective would resolve 

 lines beautifully, but its performance 

 with the Podui'a-sc3i\&, for example, 

 would be less satisfactory. 



But now, lest we be severely criti- 

 cised for our statement to the effect 

 that, to put in another form, an ob- 

 jective can be made to resolve too 

 much for its own good, we must 

 clearly state the reason for such a 

 conclusion. It is simply this : So 

 long as makers are confined to the 

 use of the glass now available, it will 

 be impossible to correct the latter 

 throughout, for central light and for 

 light of excessive angular incidence. 



As regards the value of test-plates 

 of ruled lines, they serve very well to 

 test the capability of an objective to 

 resolve lines, but since there is no di- 

 optric image from such a plate, the 



observer cannot judge whether the 

 lines are seen in their proper plane or 

 not. This point should not be over- 

 looked. A diatom is a much better 

 test, for then we have the dioptrical 

 image of the frustule — the outline or 

 the median line for example — in view 

 with the lines, and if the lines are not 

 in the same plane, or as nearly so as 

 experience indicates they should be, 

 then the objective fails to do its work 

 well. 



Not long ago several American 

 objectives were compared with two 

 by Zeiss, the test employed being A. 

 pellucida. These comparisons were 

 made by some members of the New 

 York Microscopical Society. It was 

 found that the American objectives 

 gave rather better definition of the 

 lines than the Zeiss, although this dif- 

 ference was scarcely noticeable. On 

 the other hand, the Zeiss objectives 

 showed the markings and the outline 

 of the diatom together better than 

 did the others — the two images were 

 more nearly in the same plane. 



CORRESPONDENCE. 



To THE Editor : — In the March num- 

 ber of your valuable MICROSCOPICAL 

 Journal, on page 54, under the head 

 "A New Form of Vertical Illuminator," 

 you call attention to " the ingenious de- 

 vice of Prof. W. A. Rogers," adding in 

 parenthesis, " we believe it was invent- 

 ed by him." I have a |-inch objective, 

 made by Mr. R. B. Tolles eleven years 

 ago, with a prism fitted in the manner de- 

 scribed. Mr. Tolles made one several 

 years before he made mine for Prof. R. 

 K. Browne. Please give the credit to the 

 inventor. Very truly, 



George B. Harriman. 

 Boston, March i6th. 



[In looking up the history of this device, 

 we find that Mr. Tolles should have the 

 credit of being the first to apply a prism 

 in the manner described. This he did in 

 the year 1866.— Ed.] 



MICROSCOPICAL SOCIETIES 



At a meeting of the NEW YORK socie- 

 ty, Mr. J. D. Hyatt exhibited some speci- 



