B92 Spa itt of the British Association 
A cane experimentally. Laat ACh at an azimuth of about 
r of th 
i 
aye ene 
© Fase se of. vibration of light polarized in the plane of inciden 
» dare ded ands to that voles? | in a plane lie oe a to the npn 
Barer 
wheres ke d <a a ae a determining the refractive in- 
Pack of a plate. by. means. of the displacement of a system” of in eeter ee 
for the less. refangible-colors, that the point of symmetr p> 
- apfoash to symmetry, in the system of displaced: ‘ingens i is a abated in 
advance ofthe position calculated in the ordinary way for rays of mean 
: wefrangibitity. - . Since an observer has no other guide than the symme- 
try of the’ bands in fixing’on the center of ‘the sy stem, he would thus be 
ea led to, attribute to the plate a refractive index which is slightly too ae 
"The author r has‘illustrated this subject by the following experim i) 
Be ae. ee to a theoretical error, depending oe 
‘- dispersive power.of the plat t is an extremely simple © 
{as-the author showed) of the circumstance that the ne § 
4 
ae, 
viewing the whole through a prism of moderate angle, held in front of 
the eye-piece with its edge parallel, to the fringes, an indistinct pris- 
matic image of the wires was seen, together with a distinct set of 
fringes, which lay quite at one side of the cross wires, the dispersion 
ictaaae ‘by the prism having thus occasioned an apparent displace- 
of the fringes in the direction of the general deviation. 
On é Refractive Indices #5 — Substances ; by the Rey. Prof. 
Sivicp on former occasions choise toextend the list of observed 
indices for the standard rays of the solar spectrum given by prisms 
different media, by means of an apparatus described, alon with the 
aban of the results, in my report to the British Association, 1839, 
I now beg to offer to the Association the indices in like manner obtaine 
for the four following. The rare of spikenard | received through @ 
: hett, h te ee 
perfectly pure; for the other eee i am indebted to Mr. N. 8. 5 * fe 
i The ts in each case are the mean of several repetitions. of 5 
‘ (the oils of lavender and sandal wood) the absorption © 
, 
ee 
