MICROSCOPIC ILLUSTRATIONS. 283 



without annuli, growing gradually brighter towards its 

 periphery, while a small one will give only a disc, as at 

 (6'), in focus, and exhibit annuli and a centrical spot out 

 of focus, as at (8) ; (9) will perhaps shew no difference of 

 aspect, when put out of focus, either way, with some par- 

 ticular object-glass or metal, but (8) instantly will. The 

 large globules are best to exhibit chromatic aberration, but 

 for every other purpose it may be laid down as a maxim, 

 that small globules, strongly illuminated, and put but very 

 little out of focus, are most to be depended upon. Thus, an 

 instrument will appear to be in perfect adjustment, if a 

 globule is put a good way out of focus, but far from it if 

 only very little. 



There is sometimes a discrepancy between the evidence 

 given by a dial-plate and an artificial star ; an instrument 

 will appear quite perfect, and destitute of aberration, when 

 tried only by a dial-plate, but considerably in error when 

 submitted to the ordeal of an artificial star. I believe, in 

 such a case, it may be considered that it is perfect for all 

 practical purposes, if it shews the dial-plate in a proper 

 manner in and out of focus ; for aberration and all other 

 imperfection must be considered as of a relative nature. 

 For example, an instrument may be said to aberrate when 

 applied to some intensely luminous point; but not so 

 when viewing an object of the ordinary degree of bright- 

 ness ; or it may aberrate with an opaque, but not with a 

 diaphanous body ; artificial stars must therefore rather be 

 considered as a theoretical than practical test of the good- 

 ness of an instrument, which must always be considered 

 as made not for viewing these fastidious niceties, but the 

 ordinary run of natural objects. Telescopes have been 

 made which* have shewn stars, both natural and artificial, 



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