G56 Transactions of the Society. 
The second method was an ingenious one devised by Prof. H. L. 
Smith.* He filled a hollow slide with the medium to be measured, 
and covering it with a slip he placed it in an adapter above the object- 
glass of his Microscope. It is obvious that if the refractive index of 
the medium were the same as that of the enclosing glass no more 
alteration would be caused in the focus of the entire Microscope than 
would be caused by the insertion of a piece of plane glass of similar 
refractive index and of similar thickness. If, however, the refractive 
index of the medium differs from that of the enclosing glass, the 
appliance will act as a lens with either a positive or negative focus 
according to the direction in which the refractive index of the medium 
differs from that of the enclosing glass. Fiducial points were then 
marked on the limb of the instrument, which indicated the focal 
positions of the body when a certain eye-piece and a certain objective 
were employed, and when the hollow of the slide was filled with oil 
of cinnamon (1*6), oil of cedar (1*52), glycerin (1*41), and water 
(l*33).t By interpolation the refractive indices of other substances 
are estimated. This is an excellent method, for while it is inexpensive 
and free from all complications it will nevertheless meet the wants of 
the ordinary Microscopist. For indices lying between 1*6 and 1*33 
the readings will be sufficiently accurate for all practical purposes, 
but with higher indices the ground is not so sure. 
With my former appliance I found that there was not, except be- 
tween certain limits, an increase in focus commensurate with the 
alteration in the refractive index. The plan now brought forward 
ensures an alteration of 2 in. in the focus for every change of *25 
in refractive indices from 1*0 to 3*0. 
The apparatus consists of a piece of plane glass 3x1, and the 
following lenses : a plano-convex, a plano-concave, an equi-concave, 
and two equi-convex. .The radius of all these curves is an inch, the 
concaves being hollowed out of 3 X 1 slips ; the diameter of the 
convex lenses is 3/4 in. and of the concaves 1/2 in. There is 
also provided a wooden 3x1 slip with a 3/8 hole in the centre ; 
this hole is bevelled or countersunk on one side, so that when one of 
the equi-convex lenses is placed in it the upper surface of the convex 
lens may be just flush with the top of the slip ; in other words, when 
the glass slip is placed on the top of the wooden slip the underneath 
part of the glass slip is just to touch the upper surface of the equi- 
convex lens as in G. 
On reference to the table it will be seen that these lenses are 
arranged in eight different groups to suit the different refractive 
indices of media to be measured. In the first and third the medium 
is placed between the lens and the slip, in the fifth it is placed between 
the lenses, and in the seventh between one of the lenses and the slip. 
* This Journal, 1885, p. 1066, and Amer. Mon. Micr. Journ., vi. (1885) p. 188. 
t These are the refractive indices as given in his paper. 
