416 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and do not know how it is worded, but however worded any such 

 claim must originate in a very strange misconcej)tion. 



The number of lines to an inch capable of being resolved are 

 defined by the equation 



5 = J r 



n Bin u 



Taking A. for simplicity at ^u^^^y inch (instead of • 5269 /a), and 

 u to be 180° (sin u being = 1), it will be seen that for 8 to give 

 1,000,000 lines to an inch, n — the refractive index of the immersion 

 medium (and with it the objective and the test-plate) — must be made 

 of a substance whose refractive index is 10. "What is this wonderful 

 substance — the philosopher's stone of the microscopist ? 



Or to put the same point in another way : — 



The diffraction spectra of lines 145,000 to the inch, can only just 

 be got into the back lens of a homogeneous-immersion objective of 

 1 • 50 N.A. To get in the diffraction spectra of 1,000,000 to the inch, 

 the aperture must have been not less than 10 N.A. ! How has this 

 aperture been obtained at a time too when we are congratulating 

 ourselves on having reached 1 • 47 N.A. ? 



The visibility of the diffraction spectra, so far from proving the 

 existence of lines at the rate of 1,000,000 to an inch, is conclusive 

 proof that they do not exist, and that nothing beyond 150,000 at any 

 rate could have been observed. 



High Resolving-power. — We have been referred to what is 

 termed a claim of Dr. T. S. Up de Graff to have resolved lines as fine 

 as 152,400 to the inch. Dr. De Graff's statement is, however, simply 

 that he has resolved the last band of Fasoldt's 19-band plate, and be 

 is careful to add " 152,400 to an inch, the number of lines claimed 

 by the maker to be ruled in this band " (italics in original). While, 

 therefore, fully accepting the observer's statement that the lines 

 which he did resolve were true and not spurious lines, we have, of 

 course, to wait for the demonstration that the maker's claim is correct 

 before commencing again, with clean boards, to endeavour to esta- 

 blish a theory of resolution ! The theoretical resolving power of the 

 largest apertured lens yet made (Powell and Lealand's 1 • 47 N.A.) is 

 about 141,500 lines to an inch. 



Binocular Microscopes.* — Professor E. Hitchcock, in discussing 

 the question whether there is any real advantage in binocular over 

 monocular instruments, thinks that the problem is a very difficult one 

 if we attempt to decide on theoretical grounds what effect any par- 

 ticular binocular arrangement will have when applied to the examin- 

 ation of a specified object ; to explain how much of the appearance of 

 relief is real, and how much is merely a mental impression produced 

 by the two images in the two eyes. 



He, therefore, prefers to confine the discussion to the practical 

 side of the subject. " If the question is whether there is any advan- 

 tage in a binocular Microscope in studying the form of objects — 



* Amer. Mon. Micr. Jouin., iii. (1882) pp. 45-8 (8 figs.). 



