There is no difficulty in obtaining pieces as fine as this yards long if required, or in spinning it very much finer.  There is upon the screen a single line made by the small garden spider, and the size of this is perfectly evident (Fig. 7).  You now see a quartz fiber far finer than this, or, rather, you see a diffraction phenomenon, for no true image is formed at all; but even this is a conspicuous object in comparison with the tapering ends, which it is absolutely impossible to trace in a microscope.  The next two photographs, taken by Mr. Nelson, whose skill and resources are so famous, represent the extreme end of a tail of quartz, and, though the scale is a great deal larger than that used in the other photographs, the end will be visible only to a few.  Mr. Nelson has photographed here what it is absolutely impossible to see.  What the size of these ends may be, I have no means of telling.  Dr. Royston Piggott has estimated some of them at less than one-millionth of an inch, but, whatever they are, they supply for the first time objects of extreme smallness the form of which is certainly known, and, therefore, I cannot help looking upon them as more satisfactory tests for the microscope than diatoms and other things of the real shape of which we know nothing whatever.

Since figures as large as a million cannot be realized properly, it may be worth while to give an illustration of what is meant by a fiber one-millionth of an inch in diameter.

A piece of quartz an inch long and an inch in diameter would, if drawn out to this degree of fineness, be sufficient to go all the way round the world 658 times; or a grain of sand just visible—­that is, one-hundredth of an inch long and one hundredth of an inch in diameter—­would make one thousand miles of such thread.  Further, the pressure inside such a thread due to a surface tension equal to that of water would be 60 atmospheres.

Going back to such threads as can be used in instruments, I have made use of fibers one ten-thousandth of an inch in diameter, and in these the torsion is 10,000 times less than that of spun glass.

As these fibers are made finer their strength increases in proportion to their size, and surpasses that of ordinary bar steel, reaching, to use the language of engineers, as high a figure as 80 tons to the inch.  Fibers of ordinary size have a strength of 50 tons to the inch.

While it is evident that these fibers give us the means of producing an exceedingly small torsion, and one that is not affected by weather, it is not yet evident that they may not show the same fatigue that makes spun glass useless.  I have, therefore, a duplicate apparatus with a quartz fiber, and you will see that the spot of light comes back to its true place on the screen after the mirror has been twisted round twice.