A New Befradometer. By Dr. Boyston-Pigott. 295 
following fluids (supposing it five inches deep) the bottom as seen 
through the top will appear to rise upwards, as in the table : 
Fluids. 
Displacement 
by Refraction. 
Spirit of turpentine 
Canada balsam 
Oil of cassia 
Bisulphuret of carbon 
Water 
in. 
1-62 
1-74 
1-76 
1- 94 
2- 02 
1-25 
Thus in a tube five inches long turpentine raises the image of the 
bottom (if marked with a black cross or otherwise) half an inch 
higher than the water does. 
It is plain, therefore, that if the filled tube be placed vertically 
in the axis of a long-focus microscope, capable of an adjustable 
position, marked by the readings of a vernier, the rise of the 
bottom cross can be observed. 
There are two points here to be specially attended to. 
1. The mark on the bottom flat should be as distinct as 
possible. A tine cross cut with a diamond and filled with graphite ; 
a minute bubble in the glass, capable of illumination with a mirror, 
may be recommended. It is immaterial where the mark be placed, 
inside or outside the bottom, or indeed below it, so long as it is a 
fixed point. A single globule of mercury on black velvet beneath 
the bottom of the tube would be a charming point of reference. 
2. A long-focus microscope can readily be made out of a 2-inch 
object-glass by shortening the tube. Suppose now that the tube 
be filled with water, and that 
New reading (tube filled) is . . . . 1 • 090 
Old reading (tube empty) is . , , . 2 • 347 
Difference 1 • 257 
In order now to obtain the refractive index we have only to sub- 
tract this difference from the length of the fluid column five i aches, 
which gives 3 * 743, then dividing the length five inches by this, we 
, get the refractive index 
3^ = 1-336 nearly. 
Put into general language this is : subtract the displacement of 
refraction from the length of the column of fluid, and divide the 
length by this difference, the result is the refractive index. 
Length 
Difference of length and displacement 
