156 WM. WEBB ON ''NOBERTS TESTS. 



The moment that intervening black space is annihilated by draw- 

 ing a third white line, it becomes self-evident that the three lines 

 have coalesced, and only present one line to the eye. About that 

 I think there can be no controversy. 



Having arrived at the conclusion that a line must have a space 

 on each side of it before another line can be drawn, then arose the 

 question, '' What is the space between Nobert's lines ?" I think 

 it will not be very strong presumption to assume that every micro- 

 scopist present is familiar with Dr. Jackson's Stage Micrometers, 

 having lines including spaces of the one-thousandth of an inch, or 

 with foreign stage micrometers with hundredths of millimetres in 

 which the spaces are greatly in excess of the width of the lines, 

 and the lines, comparatively coarse, because they are wanted to be 

 used with low powers, with which, if the lines were very fine, they 

 would be invisible. Ten of the lines the thousandth of an inch 

 apart would approximately embrace the field of an eighth of an 

 inch objective with an f^ eye-piece, as in my specimen numbered 1, 

 to which I shall have again to refer presently. Divide one of those 

 spaces of the one-thousandth of an inch by ten and spaces each of 

 one ten-thousandth of an inch are obtained, as in specimen num- 

 bered 2, and this No. 2 is an analogue of Nobert's first band. To 

 divide one of the one-thousandths of an inch by twenty would give 

 spaces the one twenty-thousandth of an inch, as in specimen No. 3. 

 To divide one of the one twenty-thousandths of an inch by ten 

 would give lines each of one 200-thousandths of an inch, of which I 

 have no specimen ; and, at this point of the study, I diverge from 

 the beaten path and come to the conclusion that if it be possible to 

 rule lines with clearly defined spaces they can be crossed with 

 similar lines, as in specimen No. 4, where the one four-thousandths 

 are crossed by one four-thousandths, producing squares each one 

 sixteen-millionths of an inch, which would, I believe, enclose the 

 largest human blood corpuscle. In this way lines with spaces the 

 one 200-thousandth of an inch crossing each other would produce 

 squares each the one 40,000-millionths of an inch, or, as the news- 

 papers usually misstate, such a number as the forty- billionth of an 

 inch. I claim to have some knowledge of large figures, as applied 

 to this subject, but the last one, as a ruled square, is beyond my 

 credibility. With all due deference to every gentleman who has 

 studied the subject, I respectfully suggest that beyond the first few 

 bands of Nobert's Tests there is not one containing a line properly 

 so called. The difference of opinion between gentlemen and myself 



