ABERRATION PRODUCED BY GLASS COVER. 
19 
curatelj measured and worked, and the lenses centered and ad- 
j listed with a degree of precision quite inappreciable to those 
who are not practically acquainted with this kind of work. 
32. Aberration Produced by Glass Cover. When 
achromatic object-glasses of considerable angular aperture are 
accurately tested, it is found that so perfect are the corrections, 
so completely are the errors of sphericity and dispersion bal- 
anced or destroyed, that if the glass has been adjusted for view- 
ing an object uncovered, a plate of the thinnest glass or mica 
placed over the object, sensibly disturbs the correction, produ- 
cing colored fringes and indistinctness of outline in all parts of 
the field, so that some new method of adjustment is required. 
This defect (if that should be called a defect, which arose out 
of improvement, or which was only rendered sensible by the 
great improvement given to object-glasses, as the result of Mr. 
Lister’s discovery) was first discovered by Mr. Koss, who im- 
mediately suggested a ready means of correcting it. 
The aberration referred to, which is produced upon diverg- 
ing rays by a piece of fiat and parallel glass, such as would be 
used for covering an object, will be understood by the follow- 
ing figure : 
Let GGGG be the re- ^ Fig. 6. 
fracting medium, or a 
piece of glass covering 
the object 0, and O P 
the axis of the pencil 
perpendicular to the 
fiat surfaces, O T, a 
ray near the axis, and 
O T', the extreme ray 
of the pencil incident 
on the under surface of 
the glass: then T P, 
T' K' will be the direc- 
tions of the rays in the medium, and K E, P' E', those of the 
emergent rays. 
Now it the course of these rays is continued backward, as 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
