" It will be seen by fig. 15, that when a certain Angular pencil A 0 A' proceeds from the object o, and le incident on the plane side of the first lens, If the combination is removed from the object, as In fig. 16, the extreme rays of the pencil impinge on the more marginal parte of the glue, and as the refractions are greater here, the aberrations will he greater also. Now, If two compound object-glasses have their aberrations balnneed, one Icing situated as in fib 15, and the other as in e/./. 16, and the same disturbing power applied to both, that In which the angles of incidence And the aberrations are small will not be so much disturbed as where the angles arc great, and where consequently the aberrations Increase rapidly.
':When an object-glass has its aberrations balanced for viewing an opaque object, and it is required to examine that object by trans mitted light, the correction will remain; but if it is necessary to immerse the object in a fluid, or to cover it with glass or tale, an aberration will arise from these circumstances, which will disturb the previous correction, and consequently deteriorate the definition ; and this effect will be more obvious with the increase of the distance between the object and the object-glass.
"The aberration produced with diverging rays by a piece of flat and parallel glass, such as would bo used for covering an object, is repre sented at fig. 17, where G GOO is the refracting medium, or piece of glass covering the object o; o r, the axis of the pencil, perpendicular to the flat surfaces ; o 'r, a ray near the axis; and the extreme ray of the pencil incident on the under surface of the glass : then T T' a', will be the directions of the rays in the medium, and n E, those of the emergent rays. Now if the course of these rays is con tinued, as by the dotted lines, they will be found to intersect the axis at different distances, x and v, from the surface of the glass; And the distance x Y is the aberration produced by the medium which, as before stated, interfere, with the previously balanced aberrations of the several lenses composing the object-glass. There are many cases of this, but the one here selected serves best to illustrate tho principle. I need not encumber the description with the theoretical determination of this quantity, as it varies with exceedingly minute circumstances which we cannot Accurately control; such as the distance of the object from the under side of the glass, and the slightest difference in the thickness of the glass itself ; and if these data could be readily obtained, the knowledge would be of no utility in making the correction, that being wholly of a practical nature.
" If an objecteglam is constructed as represented In fig. 16, where the posterior combination r and the middle :it have together an excess of negative aberration, and if this be corrected by the anterior combination a, having an excess of positive aberration, then this latter combination can be made to act more or less powerfully upon r And w, by making it approach to or recede from them ; for when the throe are in close contact, the distance of the object from the object-glass ie greatest ; and consequently the rays from the object are diverging from a point at a greater distance than when the combination, are separated ; and as a lens bends the rays more, or acts with greater effect, the more distant the object is from which the rays diverge, the effect of the anterior combination A upon the ether two, r And st, will vary with its distance from thence. When, therefore, the correction of the whole Is
effected for an opaque object with a certain distance between the ante rior and middle combination, if they are then put in contact, the distance between the object and object-glass will be increased : conse quently the anterior combination will act more powerfully, and the whole will have an excess of positive aberration. Now the effect of the aberration produced by A piece of flat And parallel glees being of the negative character, it is obvious that the above considerations suggest the means of correction by moving the lenses nearer together, till the positive aberration thereby produced balances the negative aberration caused by the medium.
"The preceding refers only to the spherical aberration, but the effect of the chromatic is also seen when an object is covered with a piece of glue; for, In the course of my experiments, I observed that it pro duced a chromatic thickening of the outlines of the Podurn and other delicate scales; and if diverging rays near the axis And at the margin are projected through a piece of flat parallel glass, with the various indices of refraction for the different colours, it will be seen that each ray will emerge separated into a beam consisting of the component colours of the ray, and that each beam is widely different in form. This difference, being magnified by the power of the microscope, readily accounts for the chromatic thickening of the outline just mentioned. Therefore, to obtain the finest definition of extremely delicate and minute objects, they should be viewed without a covering ; if it be desirable to immerse them in a fluid, they should be covered with the thinnest possible film of talc, as, from the character of the chromatic aberration, it will be seen that varying the distances of the combina tions Will not sensibly affect the correction ; though object-lenses may be made to include a given fluid or solid medium in their correction for colour.