294 Transactions of the Boyal Microscopical Society. 
TV. — On a New Befractometer for Measuring/ the Befr active 
Index {mean rays) of Thin Plates of Glass. Lenses, Wedges, 
and also of Fluids placed in Cavities or Tubes. By Dr. 
EoYSTON-PiGOTT, M.A., F.E.S., formerly Fellow of St. Peter's 
College, Cambridge, &c. 
{Read hefore the Royal Microscopical Society, Novemher 1, 1876.) 
Plate CLXII. 
The ordinary refractive index is taken for white light usually 
denoted by the index of the line E in the solar spectrum which 
occupies a mean position. This line, according to Angstrom's 
recent corrections, has a wave-length of 
5269-12 tenth-metres, 
which is used to denote this quantity divided by 10,000,000,000 
(ten thousand millions). And I find this to be 
48204 • 38 waves in the inch. 
For general purposes the refractive index is required not for par- 
ticular lines in the spectrum so much as the general efiect for 
light in its undecomposed state. Unless the contrary is stated 
the index is the refraction for homogeneous hght. If a bent oar 
or a raised halfcrown be observed in water, the eye has only the 
refraction of white light to deal with. The method here adopted 
refers therefore only to ordinary light considered homogeneous. 
At the same time it is applicable to any kind of monochromatic 
light. 
The popular aquarium presents a simple illustration of the 
principle employed. The bottom of the vessel appears considerably 
raised when viewed from above. Indeed it appears to have floated 
upwards nearly one-fourth of the depth. If yu, be the refractive 
index the displacement upwards will be 
- 1) - A^, 
which in the case of water is 
'1-336 - 1 -336 1 
-T336-=r336 = 4 °^"^^^- 
Now, if we suppose a glass tube furnished with a flat glass bottom 
and a flat glass cover, when the tube is filled respectively with the 
