150 



MANAGEMENT OF THE MICKOSCOPE. 



mark has been brought by the revolution of the screw-collar into 

 coincidence with the line ' covered,' it indicates that the front lens 

 has been brought into such proximity with the other two as to 



Fig. 86. 



D 



. . H 



Uncovered. ! — 



Covered. \ ~~W 



Section of an Adjusting Object-Glass. 



produce a ' positive aberration' in the Objective, fitted to neutralize 

 the ' negative aberration ' produced by the interposition of a glass 

 cover of a certain thickness. It is evident, however, that unless the 

 particular thickness of glass for which this degree of alteration is 

 suited be always employed for this purpose, the correction cannot 

 be exact ; and means must be taken for adapting it to every grade 

 of thickness which may be likely to present itself in the glass 

 covers. Unless this correction be made with the greatest pre- 

 cision, the enlargement of the Angle of Aperture, to which our 

 Opticians have of late applied themselves with such remarkable 

 success, becomes worse than useless ; being a source of diminished 

 instead of increased distinctness in the details of the object, which 

 are far better seen with an Objective of greatly inferior aperture, 

 possessing no special adjustment for the thickness of the glass. 

 The following general rule is given by Mr. Wenham for securing 

 the most efficient performance of an Object-glass with any ordinary 

 object:—" Select any dark speck or opaque portion of the object, 

 and bring the outline into perfect focus ; then lay the finger on 

 the milled-head of the fine motion, and move it briskly backwards 

 and forwards in both directions from the first position. Observe 

 the expansion of the dark outline of the object, both when within 



