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 4, (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 = n 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 

 guide, 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. 



