THE OBJECTIVE. 63 



The accompanying diagram is intended to show the 

 several lenses that enter into the construction of the 

 ordinary achromatic object-glass. A double convex 

 lens and plano-convex lens of crown-glass, and a piano 

 and donble concave lens and a miniscns lens of flint- 

 glass, are ingeniously cemented with Canada balsam 

 into a solid mass. Each objective, from the -|-inch to 

 the -jL-inch and upwards, is thus made up of at least 

 eight original lenses, the back combination of each 

 being a triplet formed of two double convex lenses of 

 crown-glass, with an intermediate double ccncave lens 

 of flint-glass. 



I cannot bring these brief observations on the object- 

 glass to a close without referring more directly to the 

 great improvement effected in balancing its aberrations 

 by the late Mr. Lister. This gentleman devised the 

 very important screw-collar adjustment, by means of 

 which the front lens of the objective is brought nearer 

 to the back lens; this at once compensates for the 

 disturbance produced by rays of light being made to 

 pass through different thicknesses of glass covers. 



When an objective has its aberrations balanced for 

 viewing an opaque object, and it is required to examine 

 that object by transmitted light, the correction will 

 remain ; but if it is necessary to immerse the object 

 in a fluid, or to cover it with glass, an aberration arises 

 from either circumstance which will disturb the pre- 

 vious correction, and deteriorate the definition ; and 

 this defect will be more obvious from the increase of 

 distance between the object and objective. 



How this very important correction is effected may 

 be further explained. If an object-glass be constructed 

 as represented in fig. 38, where the posterior combina- 

 tion p and the middle m have together an excess of 

 negative aberration, and if this be corrected by the- 

 anterior combination a having an excess of positive 

 aberration, then this latter combination can be made 

 to act more or less powerfully upon p and w, by 

 making it approach to or recede from them ; for when 

 the three act in close contact, the distance of the 

 object from the object-glass is greatest, and conse- 



