284 SUPPLEMENT TO THE 



in the most exquisite manner, while their performance on 

 the planets and other objects has been very indifferent, 

 and by no means equal to that of other instruments of the 

 same class, which failed in exhibiting stars in a satisfactory 

 manner. Engiscopes are certainly to be met with which 

 leave nothing to be wished for or desired in their per- 

 formance on natural objects, which nevertheless show 

 much imperfection in their operation on luminous points. 

 We must never forget, that as man is an imperfect being 

 himself, so all his works partake of the imperfection of his 

 organization; that, in short, absolute perfection of any 

 kind is a chimera. 



I state it as the result of my own experience, that where 

 instruments are not aplanatic, it is best for them to be a 

 little under-corrected for spherical aberration ; and, when 

 not achromatic, they are most to my taste when over- 

 corrected for colour. 



DEFECTS OF EYE-GLASSES. 



The state of these as to achromatism may be deter- 

 mined by trying them along with an object-glass previously 

 known to be achromatic ; when, if any colour appears, it 

 is of course to be attributed to the eye-glass. When tried 

 along with metallic objectives, the achromatism of an 

 Huyghenian eye-piece should be absolute and perfect ; as 

 much so, in short, as if the vision was entirely accom- 

 plished by reflection. The imperfections of eye-glasses, 

 such as seediness, bubbles, black specks, and the like, 

 when visible to the eye in looking through them, will 

 always revolve along with the revolution of the eye-piece, 



