126 Lord Rayleigh and Dr. A. Schuster. [May 5, 



its objective into two equal parts, and focussing alternately on the 

 mirror and on the image of the scale, both points could be simulta- 

 neously attended to. 



To measure the distance of the scale from the mirror the deal rod 

 used for the adjustment of the scale was cut down so as to have 

 nearly the required length. The two brass pointers attached to the 

 two ends made an angle of about 45° with the rod. One of the 

 pointers was fixed but the other could be moved round a fixed point 

 in the rod by means of a screw. As it moved the distance of the two 

 points changed, and by properly supporting the rod and leaning one 

 point against the centre of the scale at a known height from the 

 ground, while the moveable point was made to touch the centre of the 

 mirror, the distance could be accurately found. It was compared with 

 the scale itself, in order that the calculation of the angles of deflection 

 should be independent of the absolute length of a scale division. The 

 length required is the shortest line between the centre of the mirror and 

 the plane of the scale, and this can be calculated if the difference in 

 height of the centre of the mirror and the point to which the distance 

 was measured, is known. These heights were determined by means of 

 a cathetometer. The height of the centre of the objective was 

 measured at the same time ; so that all data required to find the incli- 

 nation of the normal of the mirror to the horizontal are known. 

 The following numbers were obtained; each division of the scale 

 is for simplicity supposed to be equal to 1 millim., which is very 

 nearly correct, but as has been said, its absolute value is of no im- 

 portance. 



Distance of mirror from scale in centims 252 '28 



As the position of the magnet was always read off 

 through a glass plate, a small correction equal to 

 the thickness of the glass (3 "2 millims.) multiplied 



into 1 — -, where /jl is the refractive index, has to 

 ft 



be applied. This correction is subtractive and 



equal to 0*11 



Hence, D = 252-17 



It was also found that the mirror pointed downwards, and made an 

 angle of "004 with the horizontal. A small correction due to this 

 cause will be discussed in another place. 



Torsion. — The torsion was as much as possible taken out of the silk 

 fibre, which was about 4 feet long, before the magnet was attached 

 to the mirror. The coefficient of torsion was determined by turning 

 the magnet through five whole revolutions and observing the displace- 

 ment of the magnet. It was calculated from the numbers obtained that 



