200 ELEMENTARY CHEMICAL MICROSCOPY 



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. 1 10, 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. 



