Threads up to about one twenty-thousandth of an inch in diameter may be sufficiently well measured by means of a Zeiss "4 centimetre apochromatic object-glass" and an eyepiece "No. 6" with sixteen centimetre tube length. [Footnote: The objective certainly had "4 cm." marked on it, but the focal length appeared to be about I.5 mm. only.] Sec. 86. Drawing Threads by the Catapult. The bow-and-arrow method fails when threads of a greater diameter than about 0.0015 inch are required--at least if any reasonable uniformity be demanded, and no radical change in the bow and arrow be carried out. Thus in the writer's laboratory a thread of about this diameter, within 1/10000 of an inch-13 inches long and free from air bubbles--was required. A fortnight's work by a most skilful operator only resulted in the production of two lengths satisfying the conditions. The greatest loss of time occurs in the examination of the thread by means of the microscope. Threads for galvanometer suspensions are conveniently from 0.0001 to 0.0004 inch in diameter, and are much more easily made and got uniform than thicker threads, to the production of which the catapult method applies. A reference to the diagram will make the construction of the instrument quite clear. The moving end of the quartz is attached to a small boxwood slider working on a tubular girder or between wires. The quartz is secured in position by clamps shown at A and B, and motion is imparted to the slider by a stretched piece of catapult elastic (C). An easy means of regulating the pull of the elastic is to hold it back by a loop of string whose length can be varied by twisting it round a pin. Fig. 69. [Footnote: For greater clearness of drawing, the tube carrying the slider is shown somewhat higher above the base than is convenient in practice; and the slide itself is shown too thin in the direction of the hole through it.] Since it is not permissible to allow the slider to rebound at the end of its journey, some such arrangement of breaks as is shown must be adopted. In the diagram the bottom of the slider runs on to a brass spring between the girder and the base of the appliance, and so gets jammed; the spiral spring acts merely as an additional guard. The diagram does not show the lower spring very clearly; it is a mere strip lying in the groove. A rod of quartz, with a needle at one end, is prepared as before and secured in the clamps. During the operation of fastening down the clamps, there is some danger of breaking the needle, and consequently it is advisable to soften the latter before and while adjusting the second clamp.