PROPERTIES OF LENSES. 17 



must be made for this when accurate measurements 

 of a covered object are required. 



Many methods of practically making these cor- 

 rections are mentioned in the various manuals,, but I 

 think the following will be found the easiest, and 

 sufficiently correct for all purposes. Turn the index 

 (A Fig. 12) to Zero, in other words adjust for un- 



PIQ. 12. 



covered, the object is now brought into focus. We 

 next turn the screw collar till some dust or scratches 

 on the cover-glass come into focus. The object is again 

 brought into focus by means of the fine adjustment. 



If now on turning the fine adjustment an equal 

 expansion of the outlines of the object takes place 

 when within and without the focus, the correction is 

 perfect. If the greatest expansion occurs when the 

 object is outside the focus we have gone too far, the 

 screw collar must be turned and the index moved 

 back towards or uncovered, and vice versa. When 

 the expansion is the same on both sides it will be 

 perceived that the minutest details become visible, 

 at the same time, and with the same distinctness, as 

 the outlines of the object. 



The number to which the index on the screw 

 collar points is now written on the slide for refer- 

 ence in cases of future examination. 



c 



