286 SUPPLEMENT TO THE 



to do more harm than good, and should be rejected 

 on the same grounds, or nearly so, in engiscopes as in 

 telescopes. 



The juste milieu is the double eye-glass. The defects of 

 eye-glasses as to the state of their oblique pencils, may be 

 tried by a micrometer, or ivory scale ; but recollect that 

 you must not confound the aberration of the oblique pen- 

 cils of the object-glass with those of the eye-piece, for the 

 former are irremediable, or nearly so, by the latter. On 

 this account it will be most advisable to use some object- 

 glass which has the aberration of its oblique and centrical 

 pencils nearly in the same state over the whole image, such 

 as a grooved sphere, with which a pretty accurate estima- 

 tion may be formed of the quantity of error produced by 

 the edges of an eye-glass. 



Positive achromatic eye-glasses for micrometrical mea- 

 surements, composed of two double achromatics, may be 

 examined as to their state of effectiveness by the rules 

 already laid down for trying ordinary single and compound 

 magnifiers. 



ABERRATION OF OBLIQUE PENCILS OF RAYS. 



The spherical aberration of the oblique pencils in dou- 

 ble achromatic object-glasses is very violent when their 

 angle of aperture is considerable ; (15, plate 13), represents 

 the burr produced by a globule of quicksilver in one of an 

 inch focus, and a quarter of an inch of aperture, c being 

 the effect of a large, and d that of a small globule placed 

 near the edge of the field of view, and seen in focus; (16) 

 e and f represent the appearances out of focus, e being 

 that where the greatest condensation of light is found ; 



