The President's Address, H. C. Sorhy, F.B.S., P.G.S. 3 
which he named a refractometer. His paper was subsequently- 
published, with a plate, in the ‘Monthly Microscopical Journal.’"^ 
The principle made use of in applying this instrument was the 
increase in the focal length of the object-glass of a microscope, 
caused by looking through media of different refracting power. 
The author showed that if t he the thickness of this medium, and 
d the amount of the displacement of the focus, the index of 
refraction fju may easily be calculated from the following equation : 
t 
In the instrument described by Dr. Koyston-Pigott, the amount 
of this displacement, and also the thickness of the object under 
examination, were determined by means of a micrometer screw 
fixed under the stage of the instrument, in such a manner that it 
became unsuitable for use as an ordinary microscope. 
At the time of the reading of this paper I was much struck 
with the general method employed, and in the subsequent discus- 
sion I said that probably some modification of it might prove very 
useful in studying minerals. I have now succeeded in proving this 
very completely. 
From the first I was anxious to contrive some arrangement 
that would enable us to obtain the necessary data with an ordinary 
microscope, or at all events with one so slightly modified as not in 
any way to interfere with its general use ; and I think that I have 
succeeded in accomplishing this by a very simple addition, which 
will also enable us to use the instrument for a number of purposes 
not originally contemplated. 
Practically, the application of the method I propose is very 
simple. If an object be placed on the stage of a microscope and 
the focus adjusted, on placing over it a plate of some highly re- 
fracting substance the focal length is increased, and hence, to bring 
the original object into focus, the body of the microscope must be 
moved farther from it. In order to measure the amount of this 
displacement, nothing, therefore, is required but some means for 
accurately measuring the distance over which the body of the 
microscope is thus moved. This may be roughly done with a small 
scale, accurately divided to r&ofhs of an inch ; but it is far better 
to have an attached scale and vernier, so as to be able to read 
to ToVo of inch, and to estimate half that quantity. The 
thickness of the specimen is easily measured by focussing first the 
particles of dust on the surface of the glass plate supporting the 
mineral, and then those on its upper surface. Several observations 
should be made of the position of these different planes, as shown 
by the readings on the scale, and the means taken, in order to 
* Vol. xvi., 1870, p. 204. 
B 2 
