302 SUMMARY OF CURRENT RESEARCHES RELATING TO 



shared by any scientific man, seems to depend wholly upon his belief in 

 the infallibility of his carefully concealed method of ruling them, and 

 upon his impression that he has seen the lines as high as " 150,000," 

 and upon tho equally firm impression of a few other persons that they 

 have seen all up to and including the " 200,000." These persons, how- 

 ever, admit that the higher bands furnish only passing glimpses, and 

 cannot be kept in focus and examined at leisure or shown to other 

 observers, as can bo dono with more or less ease up to " 120,000." Is 

 it possible that, after looking long and intently at the coarse and really 

 visible lines, tho retinal impressions may remain and be recognized by 

 the observer while subsequently gazing at the higher bands ? 



On another occasion, when it was claimed that all the bands of a 

 duplicate plate were resolved, and that the illumination was exceptionally 

 good and tho resolution exceptionally easy, the writer, and two friends 

 with younger eyes who accompanied him, recognized the lines of the 

 110,000 band very easily and distinctly, but failed to go further. 



" It would be evidently improper to undertake to anticipate the action 

 of the Committee as a whole, by saying exactly what should be con- 

 sidered sufficient evidence to establish the reality of certain of the lines 

 and the fact of their resolution ; but it will be noticed that the projecting 

 alternate lines must greatly aid in the task of counting a measured por- 

 tion of a band either with a micrometer or by aid of photography. It 

 can scarcely be long impossible to make a satisfactory count of the band 

 claiming to be spaced at 120,000 if it is correctly ruled, since the lines 

 really to be counted are only at 60,000. And if, which is not improbable, 

 though not yet formally demonstrated, this band should prove to be 

 successfully ruled and to be resolvable by existing lenses, a fact that 

 has been plausibly claimed but never yet really proved of any band of 

 equal fineness, then the study of the next two bands would be one of the 

 most interesting problems in the practical optics of the present day. At 

 the same time, it seems not improbable that photography may not only 

 give us an easy count of lines visible, but extremely difficult to count 

 otherwise, but may yet show the details of bands that are permanently 

 beyond the reach of direct microscopic vision." 



With regard to Dr. Ward's last remark, we should remind our readers 

 that, as we have already shown,* photography increases resolution in 

 the inverse ratio of 53 to 40, the limit being raised from 158,845 to 

 193,037 lines to the inch. 



Daylight or Lamplight for Microscopical Observation.! — Dr. W. 

 H. Dallinger, referring to the fact that Nageli and Schwendener give 

 the preference to daylight over lamplight, believing that it exerts less 

 strain upon the eye, says he suspects that the majority of English 

 observers, esj)ecially at continuous work, and with high powers, will be 

 inclined to reverse this judgment. Extremely white and intense light 

 can be obtained from good modern lamps, and, unlike daylight, it is 

 unvarying, devoid of caprice, and easy of manipulation. But this is a 

 matter, perhaps, in some sense subjective, and not of vital moment. 



Curious Interference Phenomena with Amphipleura pellucida. — 

 Mr. E. M. Nelson writes > — " I have recently observed some remarkable 

 interference phenomena in connection with photomi orographic glass 



* See this Journal, 1885, p. 968. f Nature, xxxvii. (1887) p. 173. 



