94 NOBERT’S TEST PLATE 
















Pomerania, have long been admitted by experts as the best 
known test, not only in consequence of their exceeding 
ness, but also because they are ruled to a known scale, 
because they are so close that* physicists have asserted 
it is impossible that they ever can be seen, Nobert hims 
being in this category ; and all trials of these plates, exe 
those to be herein mentioned, have resulted in failures t 
resolve the finer lines of these plates. - 
The Nobert test is a series of groups of parallel lines ruled 
on glass thus |i ill, each succeeding group being finer han 
the ee one. Different plates have a different num- 
ber of groups, ruled to different scales. The one used by 
Messrs. Sullivant and Wormly (American Journal of So 
ence, 1861) has thirty bands or groups, the coarsest having 
its lines yovv of a Paris line apart, and the finest being x yo 
each group or band being about zoo of an English ine 
width, and the whole thirty occupying a space perha 
little more than y of an inch. Now it is a difficult 
or the mind to appreciate such minute divisions of 
yet it is essential, in order to estimate a little the diffi 
of seeing such lines, to form some idea of their minut 
The average diameter of a human hair is about royo 
inch, yet in a space only one half as great in the co 
band of the Nobert plate there are seven lines, while 
30th band there are forty-five. 
The plate which I have used in the trials to be detal 
was made in 1863. It has nineteen bands, the first bé 
ruled to toys of a Paris line, and each band increasing 
five hundred, so that the 19th is +s4oo- 
The following table gives in the second column the 
tional part of a Paris line* between the lines of each 
the third column, the decimal part of a line as marked on 
~ plate by Nobert; the fourth, the number of lines to an 
lish inch; the fifth, the number of the band in a thir 
late corresponding in fineness. 

* One Paris line = .088815 of the English inch. 
