MICROSCOPIC ILLUSTRATIONS. 



the colour becomes insensible, unless the globule is illumi- 

 nated by the light of the sun. The same phenomena are 

 more faintly exhibited by a dial-plate, treated in the same 

 manner as the globule, and indeed by all objects, more or 



As it would be exceedingly difficult to give an exact pic- 

 torial representation of the phenomena of chromatic aber- 

 ration, it has not been attempted. The spherical and chro- 

 matic aberration of all single and compound magnifiers 

 may be tried in the methods already pointed out, by merely 

 looking through them at an artificial star or dial-plate, &c., 

 provided always that their focus is not so short as to pre- 

 clude the illumination of the objects in the manner indicated, 

 and that there is room to put an object sufficiently within 

 their focus to cause the. phenomena I have detailed to ex- 

 hibit themselves. But this is an operation of extreme 

 delicacy, and requires the sharpest and most practised eye 

 to attain any certainty as to the result. 



We thus obtain the characteristics of uncorrected and 

 under-corrected object-glasses and lenses both with regard 

 to spherical and chromatic aberration, and moreover those 

 which denote the absence of both, constituting a state of 

 aplanatism and achromatism. What we now want to know 

 are the distinguishing marks of objectives and lenses when 

 over corrected both for sphericity and ref Tangibility^ for 

 this state must inevitably often occur, as it is quite as easy 

 to cause the application of a correcting concave lens of flint 

 glass to operate too strongly as too weakly, and exceedingly 

 difficult to make it exactly neutralize the errors of one or 

 more convex lens, without falling short of or surpassing 

 the true mean. 



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First, then, with regard to spherical aberration, it must 



