X. — On a Simple Method of Measuring the Refractive Indices of 
Mounting and Immersion Media. 
By Edward M. Nelson, F.B.M.S. 
( Read 21st November , 1894.) 
Since my last communication * on this subject I have made some 
improvements in the plan, without unduly increasing the complexity 
of the apparatus. Before explaining them let me say how obvious is 
the necessity for some simple and inexpensive apparatus by which the 
refractive index of any medium may be readily determined. 
All those who take sufficient interest in the Microscope to master 
the optical principles upon which it is based know what an important 
part is assigned to the refractive indices of mounting and immersion 
media. Unfortunately all apparatus for the measurement of the 
refractive index is expensive, and quite beyond the reach of the ordi- 
nary Microscopist. 
An attempt to simplify the method was made by Dr. Pigott.f 
He measured the refractive index by ascertaining by means of a 
Microscope f the difference between the actual and the observed 
depth of the medium. Now if ^ is the refractive index of the medium, 
the observed depth will be - of the true depth, therefore p, may be 
determined by dividing the actual depth by the observed depth. In 
making these measurements it is necessary that the observed rays be 
nearly normal to the surface, therefore Microscope objectives which 
possess considerable aperture are not suitable for this purpose. If 
the objectives be stopped down to prevent obliquity in the observed 
rays, then there will be a corresponding want of definiteness with 
regard to the focal point. Dr. Pigott does not seem to have con- 
sidered that it is necessary that the observed ray should be nearly 
normal to the surface, for he says, “ The well-known delicacy of 
evanishment of a point under a good Microscope seemed to afford an 
exquisite test of distance.” This “well-known delicacy of evanish- 
ment ” is dependent on a large aperture, which implies obliquity in 
the observing ray, which means aberration in the medium, which 
causes an error in the measurement of the observed depth, which of 
course impairs the accuracy of the final result. 
* This Journal, 1892, p. 875. 
f M. M. J., xvi. (1876) p. 294. (It is not an original idea.) 
% The instrument shown in the cut in Dr. Pigott’s paper could not be used for 
any ordinary microscopical work, and would probably cost more than an ordinary 
refractometer. 
