240 ELEMENTARY CHEMICAL MICROSCOPY 



ence in the readings will give the displacement OO' of the image 

 of O, due to the liquid in the cell. Subtract this value from 

 A, the remainder 8 equals the distance of O'D. The refractive 



A 



index of the liquid will therefore be n = . 



5 



Instead of making scratches upon slide and cover-glass we 

 may use the condenser to project an image of some body into 

 the plane of the object slide as has been described under Method 

 4, Micrometry, page 187, employed and expressed in terms of 

 the units of the fine adjustment scale. Record the reading 

 obtained. 



Providing great care is exercised in the micrometric measure- 

 ments the determination of the displacement of image due to 

 the object slide and cover-glass may be eliminated as follows: 

 Project the image of the screen into the focal plane with no 

 slide in the field, move the slide until an observation can be made 

 through both slide and cover-glass (vertical line MI), set the 

 micrometer of the fine adjustment at zero and focus the plane of 

 the net by means of the screw adjustment of the substage condenser; 

 the displacement of the image due to slide and cover-glass has 

 thus been eliminated. Without further changing the focus of the 

 optical systems either above or below the stage, move the cell 

 containing the liquid so that an observation can be made through 

 the center of the cell (vertical line M2) . Focus up with the fine 

 adjustment; the reading of the scale will give the displacement 



O'O, .'. 5 = A - O'O and n = -. 



5 



In all cases where measurements are made by means of the 

 fine adjustment, first turn the graduated head until the pillar 

 of the instrument is raised sufficiently to allow for a liberal move- 

 ment up and down in focusing. A number of readings should 

 always be taken of the position of the focal planes and the results 

 averaged, never forgetting to lower the objective slightly below 

 the position of the sharpest focus and then raise it until the 

 image appears most sharply defined, thus avoiding the error 

 due to " back-lash." 



It is obvious that the cell must be accurately ground in order 



