in suitably recessed brass mountings, carefully main- 

 taining distances and exact coincidence with the optical 

 axis. No matter how perfect lenses may be up to, this 

 point, negligence in mounting will make them utterly 

 useless. Especially difficult is the setting of the minute 

 front lenses of the medium and high powers, the latter 

 being smaller than an ordinary pin head. The amount 

 of metal which fixes them in position is necessarily 

 very small, and the mounting thin, and although they 

 are firmly set they are extremely sensitive to any blow 

 or unusual pressure. The greatest care must also be 

 used in cementing lenses together, as the cement 

 between them is an immeasurably thin film, and if 

 there is any variation in radii of the two surfaces or in 

 their spheroidity, the cement will either fail to fill out 

 the gap or will be under a strain which will destroy their 

 usefullness. After all these processes the objective 

 must undergo a critical test under the microscope for 

 defects and if it fails in any one direction it must 

 undergo correction. On account of the many pro- 

 cesses and the several directions in which faults may 

 exist this is often very difficult. It sometimes happens 

 that one or another piece of glass possesses faults which 

 were not previously recognizable, or that there are 

 slight variations in the refractive power of one or 

 another kind of glass, either of which may be fatal to- 

 good, results ; at any rate, whatever faults may exist, 

 they must be entirely eliminated ; the final result must 



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