396 



The singular properties or, rather, phenomena shown by 

 eidola, enable the practised observer in many cases to distin- 

 guish between true and delusive appearances, especially Avhen 

 aided by the aberrameter applied to the objective to display 

 excentrical aberration by cutting off excentrical rays. 



Eidola are symmetrically placed on each side of the best 

 focal point, as ascertained by the aberrameter when the com- 

 pensations have attained a delicate balance of opposite cor- 

 rections. 



If the beading, for instance, of a test-object exists in two 

 contiguous parallel planes, the eidola of one set is commin- 

 gled with the true image of the other. But the upper or 

 lower set may be separately displayed, either by depressing 

 the false eidola of the lower stratum, or elevating the eidola 

 of the upper ; for when the eidola of two contiguous strata 

 are intermingled, correct definition is impossible so long as the 

 aperture of the objective remains considerable. 



One other result accrues : when an objective, otherwise 

 excellent, cannot be further corrected, the component glasses 

 being already closely screwed up together, a further correc- 

 tion can be applied by means of the adjustments of the 

 aplanatic searcher itself, all of which are essentially conjugate 

 with the actions of the objective and the variable positions of 

 its component lenses ; so that if ^x be the traversing move- 

 ments of the objective lenses, Sv that of the searcher, F the 

 focal distance of the image from the objective when Sw 

 vanishes, / the focal distance of the virtual image formed by 

 the facet lenses of the objective, 



The appendix refers to plates illustrating the mechanical 

 arrangements for the discrimination of eidol and true images, 

 and for traversing the lenses of the aplanatic searcher. 



The plates also show the course of the optical pencils, 

 spurious discs of residuary aberration and imperfect defini- 

 tion, as well as some examples of " high-power resolution" 

 of the Podura and Lej^isma beading, as well as the amount 

 of amplification obtained by Camera-Lucida outline drawings 

 of a given scale. — From the Proceedings of the Royal Society. 



