108 ON THE REFRACTIVE INDEX OF FLUIDS. 
thus secure the uniform exercise of the same amount of 
accommodating power. 
For the calculation of the index of refraction we must have 
the following data: 
lst. The radius of curvature of the biconvex lens = 7. 
2d. The distance between the biconvex lens and the object, 
when the latter is best seen, and air only is interpo.ag 
between the lens and its covering-plate. This distance = 
3d. The distance between the biconvex lens and theob. ~ 
when the latter is best seen, and the space between lens!&¢ts 
glass covering-plate is filled with the substance under ex and 
nation. This distance = 6. 
If now we make the required index of refraction =n, we 
have the following equation : 
eee pee (C— or ! 
ab 
This formula has been communicated to me by my colleague, 
Van Rees, and I have substituted it for that given by 
Brewster, in which the index of refraction of the biconvex 
lens is assumed as known, which, however, can be the case 
only when such a lens has been made for this express purpose 
of glass whose index of refraction is ascertained before grind- 
ing. 
The advantages of Brewster’s method are, that it is not 
only applicable to fluid bodies, but to such as are so soft as 
to admit of being pressed into the lenticular form, even when 
their degree of transparency is but feeble—a case for which 
we can provide by causing the light to traverse a thinner 
layer of the substance under examination. Different bodies, 
such as wax, pitch, opium, &c., which are in mass absolutely 
opaque, become, when pressed into a thin layer, transparent 
enough to admit of the determination of their indices of 
refraction by this method. 
The disadvantages of the procedure are the following. In 
the first place it requires the adaptation to the microscope of 
a special apparatus, consisting of an object-piece constructed 
for the purpose, and of a very accurate micrometric movement 
for measuring the distance at which the object is seen sharply 
defined. In the second place, the radius of curvature of the 
biconvex lens must be exactly known—one of the most diffi- 
cult of requirements in the case of microscopic lenses. 
Iinally, in the third place, the question arises—‘ from what 
point is the distance of the object to be measured ?” 
Brewster seems to have used the lowest point of the lens as 
his ‘‘ point de départ ”?—but this is not correct, for the true 
