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end of the screw conies to a stop, and at the other 

 goes beyond the limit of the movement and be- 

 comes inoperative. It should always be kept as 

 near as possible at the medium point. During 

 observation of the object, the milled head of the 

 fine adjustment should.always be kept between the 

 thumb and forefinger of one hand (right) turning 

 the screw in either direction to focus in different 

 planes of the object, while the other hand (left) 

 moves the object about. 



Which Eye to Use. --The right eye is generally 

 used for observations, but while the manipulator 

 may from habit be inclined to use this, it may be 

 possible that in some cases the left can be used to 

 better advantage and with less'f atigue. It is a fact 

 well known to oculists and opticians that many 

 eyes are defective, of which fact the possessors 

 may not be aware. Short or long sightedness has 

 little or no influence in viewing an object, except 

 to require a different adjustment, but so called 

 astigmatism, by which lines in a certain axis cannot 

 be seen distinctly, may influence the best results. 

 If this error is corrected by wearing glasses and 

 these are used while making observations, either 

 eye can be used. But, in order to determine 

 whether a defect exists, of which the possessor is 

 not aware, observe closely with both eyes, pre- 

 ferably an object with fine striations, such as 

 Diatomacae, to learn whether with one eye a 



