132 



STODDER, ON NOBERT S TEST-PLATE 



little more than -^'-q of an inch. Now it is a difficult matter 

 for the mind to appreciate such minute divisions- of space^ 

 yet it is essential, in order to estimate a little of the difficulty 

 of seeing such lines, to form some idea of their minuteness. 

 The average diameter of a human hair is about xrrmr of an 

 inch, yet in a space of only one half as great in the coarsest 

 band of the Nobert plate there are seven lines, while in the 

 30th band tliere are forty-live. 



The plate which I liave used in the trials to be detailed 

 was made in 1863. It has nineteen bands, the first being 

 ruled to -nnro of a Paris line, and each band increasing by 

 five hundred, so that the 19th is twtto' 



The following table gives in the second column the frac- 

 tional part a Paris line * between the lines of each band ; 

 the third column, the decimal part of a line as marked on the 

 plate by Nobert; the fourth, the number of lines to an Eng- 

 lish inch ; the fifth, the number of the band in a thirty-band 

 plate corresponding in fineness 



Has human art ever made an instrument capable of ren- 

 dering lines 112,000 to an inch visible? If not, is it possi- 

 ble to do so ? Is there anything in the laws of light which 

 renders it impossible to see lines so close, and therefore 



* One Paris line = -0888] 5 of the English inch. 



