THE DETERMINATION OF REFRACTIVE INDEX 
243 
A number of other methods for the microscopic determination 
of the refractive indices of liquids have been proposed, but these 
require specially constructed prisms, wedges or lenses, or frag¬ 
ments of glass of known index of refraction. For information 
as to methods, apparatus and accuracy the student is referred 
to the excellent paper by F. E. Wright, The Measurement of the 
Refractive Index of a Drop of Liquid. Journal Washington 
Academy Sciences (1914), 269. 
Determining Thickness by Displacement of Image. — It is 
obvious from the above discussion that if we have a transparent 
body whose refractive index we know, we can determine its 
thickness by applying similar methods. Supposing in the dia¬ 
gram,' Fig. 140, we are dealing with a solid body. Its thickness 
will be T = w O'D. In this case the value of n is known, and 
O'D can quickly be ascertained experimentally. The value for 
T thus found will be accurate within approximately 0.02 mm. 
In the absence of a cover-glass gauge, the thickness of cover 
glasses or of object slides may be thus determined: place a tiny, 
very thin drop of ink upon the upper and upon the lower sides of 
the glass plate, so that they fall almost in the same line; focus 
first upon the lower surface of the glass, using the ink spot as a 
guidOj read the fine adjustment and focus up until the upper sur¬ 
face of the slide is in focus, again read the fine adjustment; the 
difference between the two readings gives the displacement of 
image. Taking for the value of n for cover glasses and ordinary 
object slides 1.52, the thickness is readily calculated from the 
formula given above. 
Glass varies according to its composition from n = 1.52 to 
n = 1.59. For quartz, n = 1.544 to 1.553. 
