THE DETERMINATION OF REFRACTIVE INDEX 239 



adjustment of the microscope. The opening in the cell should 

 be not less than twice its depth. A depth of from 0.5 to 2 mm. 

 will be found convenient. Select a thin cover-glass of greater 

 diameter than the cell and determine its thickness. 



Scratch a very shallow mark at the center and bottom of the 

 cell, make a similar mark just outside the cell wall upon the upper 

 surface of the object slide. Make a scratch upon the cover- 

 glass near an edge. 



Fill the cell with the liquid whose refractive index is to be 

 determined. Cover with the cover-glass scratched side down, 

 being careful to exclude all air bubbles. Press gently to ensure 

 that the cell is just completely full. Remove any excess of liquid 

 with pieces of filter paper. We now have a cell in substantially 

 the condition shown in the diagram, Fig. 140. 



Place the cell upon the stage of the microscope. Focus care- 

 fully upon the upper surface of the object slide, using the scratch 

 as a guide. Read the position of the fine adjustment. Slide 

 the cell along until the projecting part of the cover-glass is in 

 the field and focus up with the fine adjustment until the scratch 

 upon the lower side of the cover-glass becomes clear and dis- 

 tinct. Record the reading of the fine adjustment. This reading 

 will be the depth DD' of the cell plus an error due to the dis- 

 placement of image resulting from the refractive index and thick- 

 ness of the cover-glass. Next focus upon the upper surface of 

 the cover-glass. The difference between this reading and the 

 previous one will give the apparent thickness of the cover-glass. 

 If T equals the true thickness of the cover-glass and / the apparent 

 thickness then T — t = x where x is the amount which must be 

 subtracted from the reading DD' to give the true depth of the 

 cell. This value will usually be slightly greater than the depth 

 as determined by a gauge. 



Now push the cell along and again focus sharply upon the 

 upper surface of the slide outside of cell and cover-glass. Read 

 the fine adjustment. Move the cell until its center approxi- 

 mately coincides with the optic axis of the microscope, focus up 

 with the fine adjustment until the scratch made at the bottom 

 of the cell is in focus. Read the fine adjustment. The differ- 



