44 PROFESSOR C. VY. BOYS ON THE 
EXAMPLE. — Measures of Fibres with Optical Compass. 
















Edge. | Fibre. Seale. | Head. Correction. 
Apparent right. . | Iie 6 3 3:45 439 | 
3 Syaieceysea Wekbl way + 255 (+ 7 078 
| —:00033 
66 KR poo) Iaeanth 3°45 545 
ss aD erga | Weft . .|) 2:55 (+4 7) 180 
Thickness. 
Apparent left edge. | Apparent right edge. 
| Left. Right. 
3°45545 | 3745439 180 545 
2:56180 2°56078 | 078 439 
‘89365 ‘89361 | — -00102 ‘00106 
89361 
| 
Mean. . *89363 centre to centre. 
— ‘00033 correction. | 
*89330 corrected value. 





Section g.—Set one microscope to see one fibre and the other in the proper groove 
in the traversing slide to see one wire. Measure, as in Section c, the distance from 
one edge of one fibre to one edge of one wire. Then move the traversing slide so 
as to measure the corresponding interval on the other side. Knowing the two 
intervals, the thicknesses of the wires and of the fibres, and the distance between the 
axes of the wires (2 R), and between the axes of the fibres (2 7), it is at once known 
to what extent the pair of fibres are eccentric with respect to the pair of wires. 
Without touching the back adjusting screw of the torsion head, screw the other two 
until one of the fibres has moved the right amount as measured by the eyepiece 
micrometer. Ifa movement of more than a few thousandths of an inch is necessary, 
it is best to re-measure the right and left intervals, and again adjust. When this is 
done the two fibres are in the same plane as the wires and exactly half-way between 
them. : 
If now the construction were perfect the torsion fibre would be in the axis of the 
instrument, and the lead balls would move centrally round it, but if either the two 
radii of movement of the gold balls 7,7 or the two radii of movement of the lead 
balls R, R differ from one another, or if the two lead balls hang from points which 
