198 ELEMENTARY CHEMICAL MICROSCOPY 



an observation may be made through the filled cell, i.e., in the 

 vertical line M 2 . The image of the net will now either be in- 

 visible or badly out of focus; having recorded the reading on the 

 fine adjustment, again focus up until the image of the net be- 

 comes sharp and clear; read the fine adjustment. This value 

 is the amount of displacement due to slide, cover glass and cell 

 contents. The difference between the first reading obtained and 

 the second gives the amount of displacement OO' of the image O 

 due to the liquid in the cell; subtract this last value from the 

 depth of the cell A; the remainder 5 equals O'D. The refractive 



index of the liquid is therefore n = 



o 



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: Pro- 

 ject the image of the grating 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 dis- 

 placement 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 plane M 2 ). Focus up with the fine adjust- 

 ment; the reading of the scale ; will give the displacement O'O, 



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



o 



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." 



