1878.] determining the limits of microscopic vision. 223 



\ 1 



the limit of which is e = -^= „ „„„„ for bkie light \ Confirmatory 



2 96000 ^ -^ 



of this was the experience of Dr Colonel Woodward of Washington, 



that he could only succeed in getting clear photographs of the 



finest lines of diatoms with the blue ray caused by passing solar 



light through a solution of ammonio sulphate of copper. And in 



the writer's experience a pale blue glass to tint the illumination 



greatly improves the experiment with the gossamer threads in 



question. 



Now, however true to the principles of the Undulatory Theory 

 of Light this expression may be, it certainly fails for dark lines. 

 For when the objectives are of the highest order of excellence and 

 .adjustment, no diffraction -line is seen by the side of the spider- 

 line miniature. But it is easily raised like a ghost of a line by 

 putting the corrections of the objectives out of order by means of 

 the "screw collars." In fact, the experiment of using only a \ 

 objective instead of the ^ gives a much larger image of about the 

 2ooWt5"^^ of an inch in diameter ; this is easily produced, and is 

 far beyond the supposed limit of microscopic vision. Mr Sorby, 

 F.E,.S., in his " President's address to the Royal Microscopic So- 

 ciety," particularly refers to the half-wave length limit for different 

 coloured rays of the solar spectrum. And the Rev. Mr Dallinger 

 has performed the feat of measuring the flagellum of monads, 

 as a mean of 250 measures, the result being less than 200 ^000 ^^- 

 Mr Sorby states in his address. No. 87, March 1876, "That the 

 interference fringes, depending upon the essential characters of 

 light itself, deserve far more considei^ation than has been given 

 to them ; that their influence has been entirely overlooked, and 

 that we cannot do better than adopt these principles in forming 

 some conclusion as to the size of the smallest object that could be 

 distinctly seen with a theoretically perfect microscope. Looked 

 at from this point of view alone, with a rfry-lens this could not 



be less than soottit^^^ *^^ '^^ vach If it were possible to make use 



of the blue end alone, lines of jy^ooo could still be seen, since their 

 shorter waves would not produce obscurity until the size was 

 reduced to X20V0 0^^ of an inch." Further on he continues, " We 

 must conclude that our instruments do enable us to see intervals 

 so small in relation to the wave length of light, that we can 

 scarcely hope for improvement as far as the mere visibility of 

 minute objects are concerned, whatever ma}^ be done to improve 

 their performances in other respects," p. 113. 



In face of these conclusions, I was fortunate enough to be able 

 to estimate the interval between two spider-lines respectively, the 



1 The lines were the 96000tli of an inch apart, as shewn in the photograph by 

 a blue solar ray (sent to the Eoyal Microscopical Society and to the writer). 



