1878.] using the Balance ivith great delicacy, $c. 



5 



the top by a frame like that of tlie table, and stayed against eacli 

 other to give firmness. The wider part of the frame, near the posts 

 E and F, is boarded over to form a table for the telescope (t, fig. 1) 

 and scale (s), by which the oscillations of the balance are observed. 

 The box containing the balance rests on two cross pieces, on the 

 narrower part, ABCD, of the frame, with the beam parallel to AD, 

 and its right end towards the telescope. 



In order to observe the position of the beam, a mirror, 1^ inches by 

 J inch, is fixed in the centre of the beam, and the reflection of a 

 vertical scale (s, fig. 1) in this is viewed with a telescope (f) placed 

 close to the scale. The light from the scale passes through two small 

 windows cut in each of the cases of the balance and glazed with 

 plate glass. The position of the beam is given by the division of 

 the scale upon the cross line on the eyepiece of the telescope. The 

 scale, which was photographed on glass, and reduced from a large 

 scale, drawn very carefully, has 50 divisions to the inch. These are 

 ruled diagonally with ten vertical cross lines. It is possible to read, 

 with almost certainty, to a tenth of a division, or -^y-oth of an inch. 

 Since the mirror is about 6 feet from the scale, a tenth of a division 

 means an angular deflection of the beam of about 3".* 



The scale is illuminated from behind by a mirror (in), several inches 

 in diameter, which reflects through it a parallel beam from a paraffin 

 lamp (J). A plate of ground glass between the scale and mirror 

 diffuses the light evenly over the scale and, by altering the position of 

 the mirror, any desired degree of brilliancy may be given to the 

 illumination of the scale. A screen (not shown in fig. 1) prevents 

 stray light from striking the balance-case. 



This method of reading — which, of course, doubles the deflection — 

 has been so far sufficiently accurate for my purpose ; that is to say, 

 the errors arising from other sources are far greater than those arising 

 from imperfections of reading. But in a long series of preliminary 

 experiments I used the following plan to multiply the deflection still 

 further. A rather smaller fixed mirror, ab, is placed opposite to and 

 facing the beam-mirror, AB, fixed on the beam, and a few inches from 

 it. Suppose the beam-mirror to be deflected from the position BL. 

 parallel to ab, through an angle, 9, to the position AB. If a ray, PQ, 

 perpendicular to ab strikes AB at Q, it will make an angle, 9, with 

 QM, the normal at Q, and will be reflected along QR, making an 

 angle, 29, with its original direction, and therefore with the normal 

 RO, at R, when it strikes it. If it be reflected again to AB at S, it 

 will make an angle, 30, with the normal SN, and the reflected ray, 

 ST, will make an angle, 4#, with the original direction, PQ, of the 

 ray. It may be still further reflected between the two mirrors, if 



* The numbers on the scale run from below upwards, so that an increase in the 

 weight in the right hand pan is indicated by a lower number on the scale. 



