. The microscope and its revelations. that they will be refracted to Rand P (shown by the dotted lines of the figure). On their emergenceinto air they will be again refracted in a direction parallel to theirfirst path, and will enter the front lens of the objective at .thepoints M and IS. Now as MR and N P, produced, meet in Y, it follows that, sofar as the objective is concerned, the rays M R, X P might havediverged from the point Y. Similarly, by tracing two of the less divergent rays from O theywill be made by the refraction of the cover-glass to appear as ifthey diverged from X. Therefore,
. The microscope and its revelations. that they will be refracted to Rand P (shown by the dotted lines of the figure). On their emergenceinto air they will be again refracted in a direction parallel to theirfirst path, and will enter the front lens of the objective at .thepoints M and IS. Now as MR and N P, produced, meet in Y, it follows that, sofar as the objective is concerned, the rays M R, X P might havediverged from the point Y. Similarly, by tracing two of the less divergent rays from O theywill be made by the refraction of the cover-glass to appear as ifthey diverged from X. Therefore, in consequence of the cover-glassthe objective has to deal with rays radiating apparently from tn;-, dis- c 2 20 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS tinct points, X and Y. If there were no cover-glass all the rays woulddiverge from O, and then the objective would require to be perfectlyn/t/tntnfic. This word (derived from a = privative, and TrXaraw, towander, free from wandering or error) means, as used by opticians,. FIG. 21.—The effect produced by a cover-glass on the corrections of an object-glass. that all the rays passing through a lens system are brought to an identi-cal conjugate focus, as shown in fig. 22. But as affected by the cover-glass the marginal rays diverge, apparently, from a focus, nearer theobjective than the central rays ; therefore the objective, to meet thiscondition, must be what is called under-corrected ; a condition pre-sented in fig. 23, so as to focus both these points at once. Here the
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