[Illustration:  Scale of 1000ths of an inch for Figs. 1 to 7.  The scale of Figs. 8 and 9 is much finer.]

[Illustration:  FIG. 1.]

[Illustration:  FIG. 2.]

[Illustration:  FIG. 3.]

The first photograph (Fig. 1) is an ordinary hair—­a sufficiently familiar object, and one that is generally spoken of as if it were rather fine.  Much finer than this is the specimen of copper wire now on the screen (Fig. 2), which I recently obtained from Messrs. Nalder Brothers.  It is only a little over one-thousandth of an inch in diameter.  Ordinary spun glass, a most beautiful material, is about one-thousandth of an inch in diameter, and this would appear to be an ideal torsion thread (Fig. 3).  Owing to its fineness, its torsion would be extremely small, and the more so because glass is more easily deformed than metals.  Owing to its very great strength, it can carry heavier loads than would be expected of it.  I imagine many physicists must have turned to this material in their endeavor to find a really delicate torsion thread.  I have so turned only to be disappointed.  It has every good quality but one, and that is its imperfect elasticity.  For instance, a mirror hung by a piece of spun glass is casting an image of a spot of light on the scale.  If I turn the mirror, by means of a fork, twice to the right, and then turn it back again, the light does not come back to its old point of rest, but oscillates about a point on one side, which, however, is slowly changing, so that it is impossible to say what the point of rest really is.  Further, if the glass is twisted one way first and then the other way, the point of rest moves in a manner which shows that it is not influenced by the last deflection alone:  the glass remembers what was done to it previously.  For this reason spun glass is quite unsuitable as a torsion thread; it is impossible to say what the twist is at any time, and therefore what is the force developed.

[Illustration:  FIG. 4.]

So great has the difficulty been in finding a fine torsion thread that the attempt has been given up, and in all the most exact instruments silk has been used.  The natural cocoon fibers, as shown on the screen (Fig. 4), consist of two irregular lines gummed together, each about one two-thousandth of an inch in diameter.  These fibers must be separated from one another and washed.  Then each component will, according to the experiment of Gray, carry nearly 60 grains before breaking, and can be safely loaded with 15 grains.  Silk is therefore very strong, carrying at the rate of from 10 to 20 tons to the square inch.  It is further valuable in that its torsion is far less than that of a fiber of the same size of metal or even of glass, if such could be produced.  The torsion of silk, though exceedingly small, is quite sufficient to upset the working of any delicate instrument, because it is never constant.  At one time the fiber twists one way and another time in another, and the evil effect can only be mitigated by using large apparatus in which strong forces are developed.  Any attempt that may be made to increase the delicacy of apparatus by reducing their dimensions is at once prevented by the relatively great importance of the vagaries of the silk suspension.