Limits of Microsco^ie Vision. By Dr. Roijston-Pigott. 1 1 



great distance. I myself knew a friend who could see with the 

 naked eye Jupiter's satellites, and dot down their position though 

 ignorant of astronomy. If a simple organ of sight can distinguish 

 such objects as subtend only a second or two, it would seem strange 

 that modern glasses can only show objects presenting many seconds 

 to the eye at the last visual image formed in the eye-piece. 



In the following observations I shall endeavour to substantiate 

 a fact apparently irreconcilable with the results of the now famous 

 formula.* 



In point of fact, the opinion has now become established both in 

 Europe and America, that Nobert's lines 112,000 per inch (or lines 

 of that size) are the closest that can be seen ; and that the law 

 enunciated in the footnote forbids the hope of farther advance in 

 minute definition. 



Now, considering the readiness with which a fine horsehair can 

 be distinguished against a light cloudy sky, as also spider lines at 

 several feet distance, I determined to mount upon glass several spider 

 threads and measure their diameter by means of Browning's spider- 

 line recording micrometer. After many trials, I found the smallest 

 of these measured l-35000th in diameter (Fig. 4). 



I measured them by means of Powell and Lealaud's magnificent 

 |- dry lens. On this spider thread i could perceive irregularities, 

 nodules, and marks ; but the general thickness was remarkably true. 

 Some others measured tg^h^oj ts^oo- ^'^^ coarse agglomerations, 

 cord-like, were as thick as tine spun glass ^oV^th. 



It then occurred to me to make a novel use of the " Aerial Micro- 

 meter " formerly described by me, consisting of the " Browning " 

 inverted beneath the sub-stage, as also placed in a reversed position 

 (see Fig. 3). 



The law established contains two remarkable elements: the 

 kind of light, i. e. the length of the wave, and the aperture of the 

 objective. For blue light (wave \ = 53000 per inch) intermediate 

 between blue and indigo, this, with an aperture of 150", would give 



, nearly = 



53000 X 2 X -960' •' 102000 



* This is thus stated : — 



If € represent the smallest interspace recognizable between two bright lines or 

 flisks, on the condition that the diffraction fringe of one does not overlap that of 

 its neighbour ; and 



If X be the length of the wave of light under consideration, which for mean 

 rays equals ^-gxr^ of '^^ ii^^'i ! ^°*i 



If o be the semi-aperture of the objective, 



- (when aperture = 180-), 



and 



= l-96590th (when aperture = 150°). 



