240 ELEMENTARY CHEMICAL MICROSCOPY 
ence in the readings will give the displacement 00^ of the image 
of O, due to the liquid in the cell. Subtract this value from 
A, the remainder d equals the distance of O'D. The refractive 
A 
index of the liquid will therefore be w = y. 
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 
shde 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 
A 
O'O, 5 = A - O'O and n ^ 
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 
