107 
Mopes of Dretrerminine, by the Use of the Microscorz, the 
Rerracrive Inpex of Fiurps. 
(Freely translated from the Dutch of Professor Hartine, of Utrecht. 
By Wixtiam Rosertson, M.D., F.R.C.P.E. See ‘ Het Mikroskoop,’ 
Tweede Deel, b. 200.) 
A KNOWLEDGE of the laws which the rays of light observe 
in their course through refracting media enables us, with the 
help of the microscope, to determine the index of refraction 
of certain substances, to which, on account either of their 
small quantity or of their insufficient transparency except 
when in layers of extreme thinness, the ordinary methods are 
inapplicable. 
I. Sir David Brewster’s Method. 
It is many years since Brewster first used the microscope 
for this purpose. His mode of procedure is described in his 
‘ Treatise on Philosophical Instruments,’ Edinb., 1813, p. 240. 
He uses a compound microscope, the object-glass of which is 
a biconvex lens, with sides of equal curvature, and of consi- 
derable focal length. This lens is firmly fixed in the lower 
extremity of a brass ring, which is to be filled with the fluid 
whose refractive power is the subject of examination. The 
upper opening of the brass ring is then to be closed by laying 
on it a circular glass plate with parallel surfaces. The con- 
tained fluid now forms a plano-concave lens, the concavity of 
which rests on the upper side of the biconvex glass lens. 
The object-glass is thus converted into a plano-convex com- 
pound lens, resembling an achromatic combination of flint- 
and crown-glass, but with this difference, that in the former 
the convex surface is directed downwards and the flat surface 
upwards. 
When the biconvex is thus converted into a plano-convex 
lens, its focal length becomes of course considerably aug- 
mented ; and in like manner the distance at which an object 
must be placed in order to be clearly seen through the mi- 
croscope becomes greater. 
That the eye may in the course of a series of observations 
be as nearly as possible in the same state of accommodation, 
Brewster recommends the use of an eye-piece with a wire 
or glass fibre crossing its field, to form a distinct image on 
the retina at the commencement of each observation, and 
