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Ow Certain Imperfections and Tf.sts 0/ Object-glasses. 

 By G. W. Royston-Pigott, M.D., M.E.C.P., M.A., 

 F.R.A.S., late Fellow of St. Peter's College, Cam- 

 bridge. (Received Sept., 1869.) 



The actual diameter of the least circle of aberration 

 caused by lenticular vision is the real gist of the much- 

 debated question of " aberration " and imperfect definition. 

 The performance of the eye-piece is altogether secondary and 

 inferior to that of the objective, whose errors are multiplied 

 by it, as well as the ratio of the distances of the final con- 

 jugate foci from the posterior lenses of the objective. 



This circle or ring, being the smallest space through 

 which the focal pencil passes, is seldom so reduced as to 

 leave no traces in the highly developed image presented to 

 the eye. 



Omitting here to dilate upon the terms now so vaguely 

 used, as " resolution," " penetration," and " definition," I 

 may be permitted to enumerate a few points worthy of con- 

 sideration, as they have occurred to me during the last 

 twenty-five years. 



Under the use of very high power every free edge of an 

 object, and every isolated point, exhibits an umbra and 

 penumbra exactly representing the diameter of the least 

 circle of aberration generated by the final objective re- 

 fractions. 



Every object being an assemblage of such points exhibits, 

 under high power, similar aberrating shadow, principally 

 visible at the sharp borders and edges ; this shadow or 

 penumbra depending upon the aberration, and being inde- 

 pendent of the size of the object considered as an assemblage 

 of points. After a multitude of experiments, I conclude 

 that — 



This shadow can be considerably diminished — 



(a) By limiting the aperture of the illuminating pencil. 



(b) By reducing the aperture of the objective. 



(c) By further correction and better approximation to 

 aplanatism of the objective itself. 



(d) By viewing objects directly, without a covering glass, 

 properly adjusting the position of the front lenses. 



(e) By the use of immersion lenses, destroying the aber- 

 rating effects of a plate of air. 



(/) By carefully searching the axis of the instrument for 

 a position of minimum aberration of the conjugate foci. 



{(/) By employing direct rays, from a radiant point, free 



