IGO WM. WEBB ON " NOBERT's TESTS." 



of tliis refraction of liglit is strongly coDfirmatory of the accuracy 

 of my views. 



After much thought, I have come to the following conclusions, 

 which I now submit, not as absolutely correct, but for the purpose 

 of assisting other students in arriving at their own conclusions. 

 For what they may be worth, I respectfully submit the following — 

 That a micrometer with lines the one 200-thousandth of an 

 inch apart ruled on glass is an absolute impossibility. 



That if it be possible to rule lines themselves of the width of the 

 one 200-thousandths of an inch, to make them definable, there must 

 be a clearly defined line between them. 

 And, 

 A clearly defined line in the same plane of observation. 

 That beyond the first few coarse bands of M. Nobert's Tests, 

 there is not, properly so called, a single line. 



That in the finest bands, except at their extreme sides, there is 

 not half a line. 



That in the finest bands the only thing certain, except the edges, 

 is the uncertain polarised aerial lines. 



That the microscopical world has been pursuing a phantom, and 

 adopting a fallacy. 



That polarisation of light in the examination of these and 

 analogous tests is a deceitful servant of the microscopist. 

 That oblique illumination is another deceiver. 

 That if M. Nobert were to attempt to fill his incisions with black, 

 his finest bands would be merged each into one black line of the 

 breadth of each particular band. 



That a test must be a known thing which some powers will 

 either disperse or fail to define, as in the case of a spectacle vendor, 

 who places before an intending purchaser's eyes, words printed in 

 types of different sorts as a known test of visual powers. 

 That there are no tests so reliable as plain opaque lines. 

 That of plain opaque lines, there are none so reliable as a known 

 measured congeries of contorted lines, as in microscopic writings, 

 where the transmitted rays are partially shut off by the black, and 

 in which the rays transmitted being transmitted by direct illumina- 

 tion their definition is not interfered with, such rays becoming 

 parallel rays, passing out at right angles with the surface of the 

 glass, the unalterable law of natural optics being that the angle of 

 incidence and the angle of reflection are equal. 



