422 Dr. Goring on AchromaiiG Microscopes. 



little can be seen : in the same manner, the fog resulting from 

 spherical aberration consists of false and wandering rays not 

 brought to a focus ; but where there is little light to be scat- 

 tered, little fog can be generated*. 



It, however, must never be forgotten that it is no merit in 

 a compound microscope to show objects which are to be seen 

 much better with its object-glass alone, than with the help of 

 the body attached to it. It is no wonder that the vilest ma- 

 chines ever turned out of the glazier's hands should show test- 

 objects when their object-glasses have -J^y, -fj, -^q, or of an 

 inch focus with large apertures, because such lenses can by 

 their own unassisted natural power show everything which 

 can be seen by microscopes of any kind whatsoever. 



Now Mr. Tulley's curves are published, doubtless, every 

 optician will have achromatic object-glasses for microscopes 

 as plentiful as blackberries, and at very reasonable rates too ; 

 a brace of convexes of crown glass or plate will have a con- 

 cave of flint inserted between them ; the composition will be 

 called achromatic, and it will be very hard if the uncor- 

 rected aberration cannot be conquered by a reduction of the 

 aperture, since that of a common lens may. A stop of very 

 small aperture will be placed within the tube to ejffect this 

 valuable purpose. Thus, this noble invention will get a bad 

 name, and such achromatic object-glasses will be damned by 

 all men of discernment as no better, or perhaps worse, than 

 common lenses, just as Mr. Tulley's first attempts were by 

 Mr. Troughton; but if my readers will do me the honour to 

 attend to this disquisition on test objects, it will be in their 

 power to examine the pretensions of all microscopes whatso- 

 ever, with infallible accuracy, and the unblushing effrontery 



* The aberration of all small lenses must, of course, be very small, 

 when considered as a measured quantity, compared with that of larger 

 ones ; but as it will always bear the same ratio to their focus with any de- 

 terminate angle of aperture, I do not think their superior distinctness to 

 be owing to this circumstance, but purely to the rarity or thinness 

 of their light, or the few rays which have to co-operate in the production 

 of their images ; in confirmation of which position, I may observe, that it 

 is notorious to all persons conversant with optical instruments, that the 

 chromatic and spherical aberration of both telescopes and microscopes 

 becomes much more sensible when objects are viewed by them in a strong 

 and intense illumination, than when seen in a dull and faint one. 



