144 
DE. EVEEETT ON THE EIGTDITY OF GLASS. 
In the present case the ratio m 1 : m 2 is about 1 : 3, and the values of (3 for the centre 
of the scale in the position occupied by the apparatus on July 17th and 18th were, 
For torsion, &=:-&, /3 2 =^j ; for flexure, /3 2 =- 3 \. 
Hence we find by the above formula that 
T is too great by -0025 T, 
F „ „ -0013 F. 
2nd. Correction for change of distance between mirror and telescope. If the mirror 
is moved parallel to itself to or from the telescope by the amount b, and if <p denote the 
angle between incident and reflected ray (or rather between their projections on a vertical 
plane perpendicular or parallel to the rod), the change produced in the scale-reading is 5<p. 
In the present case this correction was found to be insensible. 
For the total corrections applicable to the observations of July 17th and 18th, we' 
have therefore 
+ •0208 T--0025 T=+-0183 T, 
+ •0172 F--0013 F= + -Q159 F, 
T . 
and the resulting correction of the quotient p is 
(•0183— -0159)|=-0024|- 
This correction reduces the values of Poisson’s ratio derived from the observations of 
those days to *225, *233, -224. 
For the observations of July 13th, 14th, and 16th, the correction of F is the same as 
above. As regards the optical correction of T, a distinction must be made between the 
observations marked I ( a ) (b) and those marked II (a) ( b ). In the former, the central 
portion of the scale was on the cross wires of the telescopes, in the latter a portion of 
the scale nearly vertical over the mirrors. The optical correction for T applicable to 
the centre of the scale on the date in question was — -0089 T, and we shall apply this 
correction to the values I (a) (b), so that the total correction of T for these values will be 
+ •0208 T— -0089 T= + -0119 T, 
T 
and the corresponding correction of p will be 
(•0119— *0159) p= — -004 p? 
which reduces the value ‘246 of Poisson’s ratio to *241. To the values 11 {a) (b) we 
shall apply the same corrections as to the observations of July 17th and 18th, and the 
value ’220 of Poisson’s ratio is thus reduced to ’223. The corrected values of Poisson’s 
ratio -225, - 233, -224, -241, -223 give the mean value - 229 ; and it will be noted that 
every one of the five determinations (whether corrected or uncorrected) is less than one- 
fourth. 
The five determinations of T and F uncorrected and corrected, are given below. The 
