Noherfs Test-Plates. By W. Wehh. 173 
line as much on each side as the depth of the etching ; secondly, 
because, independently of the peculiarity of the etching, the lines 
were first of all ruled by the act of clearing away the wax coating 
to expose the glass to the action of the fumes ; and thirdly, because 
the line thus ruled must of necessity be very much finer than it 
becomes when etched. 
The present writer publicly thanks Dr. J. J. Woodward for his 
handsome present, in 1874, of wonderfully clear photographic 
prints of the first seven bands of a Nobert's plate, of a Lord's 
Prayer blackened in and fixed down with balsam, and a Lord's 
Prayer mounted dry and unblacked, in which the letters were less 
than the twenty-eight millionth part of an inch. The Quekett 
Microscopical Club also received similar photographs. A comparison 
of the three photographs affords visible proofs of the accuracy of the 
theory that the spurious lines alluded to by Mr. Eogers and by 
many other writers, are due to polarization alone, because, in the 
photograph of the Lord's Prayer, blackened in, and taken with a 
Powell and Lealand's immersion eighth, the surface of the glass 
and the black letters were equally and only in focus at one and the 
same time, thereby producing a clearly defined Lord's Prayer with- 
out any " spurious lines" because there was no possibility of pro- 
ducing any other than the normal refraction upon the entrance of 
the rays ; while the photographs of the unblacked Lord's Prayer 
and of Nobert's lines both gave spurious lines, evidently from the 
same cause, but under different aspects, inasmuch as the Lord's 
Prayer is equally cut throughout, and therefore all in one focal 
plane, while the lines of Nobert vary in depth, the one band from 
another, thus precluding the possibility of each band being in focus 
at the same time — e. g., when the seventh band is in focus, being at 
the surface of the glass, the first band, being much deeper cut, is 
out of focus, except at the very edge of each side of every line, 
giving a broad white line, having on each side of it a broad densely 
black line, the true line being white in consequence of the ^th 
being incapable of penetrating the depth of the cut beyond the 
mere entrance, and leaving in the focal plane a space of the cut 
wholly without substance. The intangible black lines, although 
very palpable to the mind, are easily demonstrated to be due to 
polarization, if a gentleman will use his polariscope with a clean 
plain glass slip upon the stage, when it will be found that at one 
period of the revolution of the polarizer the glass will transmit 
only rays of light closely resembling the invisible rays at the end 
of the solar spectrum, except that they are not violet but black, 
and are precisely the same as in the photograph, and produced by 
the same means, only altering the position of the prism and placing 
it (one to every line) by means of the V cut in the body of the 
slip ; each prism thus placed 'polarizing the light hy again bending 
