in Ruling his Test-Plates. By William A, Rogers. 
87 
this space, then the width of the line under examination is Y^iirTj X t3- 
= of an inch ; a result which is obviously within the 
truth, especially if the line in the eye-piece is made a shade larger 
than the line under the objective. Tested in this way, the lines of 
Nobert's 19th band are about yg^Voo i^^^ width. The 
photographs made by Dr. Woodward seem to give a little greater 
value. The finest lines I have succeeded in ruling are about 
1 6 0^0 0 0 i^^^ width. These values are substantially the 
same as those given by Dr. Koyston-Pigott, as representing the 
ultimate limit of visibility under the niicroscope. The smallest 
angle at which an object can be distinctly seen is stated by him to 
be 6", while other writers place it as high as 60", or even 120". 
Even the smallest value named is much too large. I will at any time 
undertake to rule a single line, -^-(j^o-^ of an inch in breadth, which 
can be seen at the distance of seven inches from the eye. This 
corresponds to an angle of about 1". In this case the line is filled 
with plumbago, but if it is reflected from a silvered surface it can 
be easily seen at the distance of eleven inches from the eye. Com- 
paring minute particles of matter which can be seen under a Tolles' 
—th. objective with those which can be measured, in the way indi- 
cated above, there is every reason to suppose that the limit ot 
visibility falls beyond y-g-oVo o- of inch. It is quite possible that 
the conclusion reached by Sorby, that the microscope has already 
reached the limit of its power in separating lines whose distance 
apart is equal to one-half of a wave-length, may be found to be 
justified by future observations. It is certain that no lines heyond 
Nobert's 19th band have ever been resolved. The great difficulty 
in distinguishing true from spurious lines has caused more than 
one skilful microscopist to doubt whether the resolution has been 
certainly carried as far as that point. But that light is " of too 
coarse a nature " to enable us to see particles of matter as small as 
2 0 0 0 of an inch, is a conclusion which can be refuted without the 
slightest difficulty. . 
How are Noherfs Finest Lines produced ? 
In trying to answer this question, I shall give the results of 
four distinct lines of investigation. Neither of these furnish con- 
clusive evidence, but they are all suggestive of possibilities. 
I. I have already stated that there is strong evidence that they 
are ruled with a diamond having a knife-edge. To this is added a 
fact derived from my own experience, and confirmed by a trial of 
several months, viz. that when a diamond, having a polished 
hnife-edge, is set slightly inclined to the direction of the lines ruled ^ 
its ruling qualities improve with use. The diamond with which 
bands of 50,000 Hnes to the inch were first successfully ruled 
