Liinifs of Microscopic Vision. By Dr. Roxjston-Pigott. 19 



diality. It is superior to all others for detecting residuary errors, 

 and when these are nearly compensated the miniatures of spider 

 lines of any size are portrayed with enchanting precision. 



To sum up : — The whole question of minute vision is the least 

 visual angle first of naked vision, and secondly in instrumental 

 vision. 



It can hardly be expected that any Microscope, especially if 

 connected with miniature apparatus, involving the total use of 

 some twenty lenses arranged as nearly as possible with one conti- 

 nuous optical axis, — that any iMicroscope, I say, can ever equal the 

 simplicity of human vision. But then, with the unassisted sight 

 we can easily determine the limits of vision by receding from the 

 object, and so making the visual angle smaller and smaller until the 

 hair vanishes. This we may call the vanishing angle 6. 



Now the art, if I may so speak, of making very minute objects 

 visible, may be applied by my method to render them distinctly 

 visible as they get smaller and smaller as miniatures, and at last 

 reach the vanishing limit. 



But to my eye, which is, I must confess, the worse for these 

 experiments, hues can be formed under the Microscope which also 

 by lowering the ocular power, or diminishing the miniature, 

 resemble (I will not say absolutely identify themselves with) the 

 vanishing phenomena of naked vision. 



When I see spider lines sharply defined become beautifully less, 

 and give one the same appearance as a hair upon a window-pane, 

 vanishing as its visual angle reaches the limit, I am bound to 

 believe, nay be assured, though against all modern belief and theory 

 apparently, that I do see these exquisitely small lines just on the 

 point of evanishment at a very small visual angle indeed. Anyone 

 with ordinary sight can see a human hair on a window-pane 

 against a moderately white sky at a distance of two feet and a 

 quarter. This is an angle of 20 seconds. 



At five feet it is Nine seconds. 



„ ten feet it is Four and a half seconds. 



„ twenty feet it is ,. .. Two and a quarter seconds. 



Now, on comparison of the minute lines exhibited by me 

 microscopically, the hair lines appear equally small in each mode, 

 either by viewing them on a window-pane at a yard off, or in the 

 microscope diminished fifty times, and then sufliciently enlarged. 

 The irresistible conclusion from this comparison is that the eye can 

 discover a minute hair line either on the window-pane or in the 

 apparatus exhibited, at certainly a smaller angle than 20 seconds. 

 In other words, the minute microscopic image appears as small as 

 a hair several feet off, according to the acuteness of vision. 



The highest experimental proof by comparison is thus strongly 

 in favour of a line sharply and clearly defined, subtending an 



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