330 SCIENTIFIC APPARATUS. 



ference of the lenses, since otherwise there can be no perfect focus, 

 and in every position the image must be somewhat indistinct. 

 These various difficulties have been, to a very considerable extent, 

 overcome by using compound lenses, consisting, usually, of a 

 double convex lens of crown glass cemented with Canada balsam 

 to a plano-concave lens of flint glass. When the curves are pro- 

 perly related to the dispersive power of the two different kinds ot 

 glass, the unequal refraction of the two extreme ends of the spec- 

 trum may be almost overcome, but still the so-called irrational 

 dispersion of glass makes it nearly impossible to obtain an 

 image absolutely free from colour. The spherical aberration 

 may be to a great extent overcome by a proper combination ot 

 several of the above-named compound lenses of different sizes, 

 differently corrected. All these difficulties are greatly diminished 

 by reducing the aperture of 'the lenses and using only their more 

 central portions, but in that case the value of the object glass is 

 greatly diminished from other causes. What is desirable in the 

 abstract is to have an object glass of large aperture, which does 

 not approach too near to the object, and so constructed that 

 when combined with the eye-piece it may give a perfect image 

 free from false colour. The attainment of all these advantages 

 is so extremely difficult in the case of high powers, that even the 

 best object-glasses are little more than the best possible com- 

 promises between opposing qualities, and it becomes a question 

 whether lenses of high power should not be designed and made 

 each for a particular class of objects, since a quality which is of 

 paramount importance in one case is not in another. Thus, for 

 instance, in examining some objects it may be far more important 

 to be able to obtain moderately good definition through a con- 

 siderable thickness of substance, than through a very small 

 thickness to separate lines or markings at very close intervals, 

 whilst it may be the reverse in the case of other objects. 



Before passing from the question of object-glasses, it may be 

 well to call attention to one or two points not yet noticed. 





