THE DETERMINATION OF REFRACTIVE INDEX 197 



Method. Cement upon a thin object slide of clear glass, a 

 ring, whose top and bottom are ground true and parallel. The 

 cell thus formed should be approximately one millimeter deep 

 and several millimeters in diameter. Place in the cell the liquid 

 whose refractive index is sought and cover with a thin cover glass 

 of greater diameter than that of the cell, as shown in Fig. no. 



Determine the thickness of the liquid layer by means of the 

 graduations on the fine adjustment as follows: focus carefully 

 upon the upper surface of the object slide. Read the position 

 of the fine adjustment. Slide the cell along until the projecting 

 cover glass is in the field and focus upon the under side of the 

 cover glass and record this value. This will give the depth of 

 the liquid plus an error due to the refraction of the cover glass. 

 Next focus upon the upper surface of the cover glass. The dif- 

 ference between the last two readings will give the apparent 

 thickness a of the cover. The true thickness x of the cover 



x 

 can now be found from the equation 1.52 = - where 1.52 is the 



Of 



refractive index of glass, and x a will be the value to be sub- 

 tracted from the thickness of the liquid layer found above in order 

 to obtain the true thickness. Call this corrected thickness A. 



Now push the preparation along so that only the object slide 

 appears in the field. Focus sharply upon the upper surface. 

 Without disturbing the adjustment push the slide to one side so 

 that it is no longer in the field and by means of the Abbe con- 

 denser project the image of a net ruled scale into the plane of the 

 upper surface of the object slide. (See page 156.) This is done 

 by adjusting the Abbe condenser until the scale is very sharp 

 and clear in the microscope. Under no circumstances must the 

 focus of the microscope be changed. Push the slide into the 

 field so that an observation may be made outside the cell but 

 through both slide and cover glass, that is in the line MI. The 

 image of the net rulings will be faint and hazy. Read the fine 

 adjustment and focus up by means of it until the rulings become 

 as sharp and clear as in the first place. There is thus obtained 

 the amount of the displacement of image due to slide and cover 

 glass. Without changing the focus, move the preparation until 



