1879.] Microscopical JResea?x7ies in High Powev Definition. 165 



These difficulties are principally created by overlapping images — 

 due partly to residuary aberrations both spherical and chromatic — 

 partly tc the effects of diffraction, caused by brilliant illuminations of 

 spurious disks of light — partly to the constant development of Eidola 

 or false images, which vary the loci of their development according to 

 the nature of underlying structures and according to the object- 

 glasses being over or under corrected ; and they are partly, and indeed 

 very considerably, created by the use of excessively large angular 

 apertures. 



The paper discusses also, the relative effects on visibility, of large 

 and small angular apertures in objectives. 



It shows that the black margins or black marginal annuli of refract- 

 ing spherules, constantly displayed by low aperture glasses, are 

 attenuated gradually to invisibility as the glasses employed are 

 endowed with the largest apertures. That the black margins also of 

 cylinders, tubules, or semi- tubules suffer similar obliterations. And 

 that, in consequence, innumerable minute details are concealed or 

 destroyed till the aperture is sufficiently reduced. 



That minute refracting bodies obey the laws of their refrangibiiities 

 and display beautiful phenomena, discoverable by transcendent 

 powers of definition; but totally unseen by inferior compensations. 

 And that, in consequence, the so-called achromatism of modern glasses 

 is an illusory approximation to correct vision. 



Examples are given of molecular structures, varying in form, trans- 

 lucency, and refrangibility, in which natural pencils are caught and 

 displayed in the order in which, as in a rain drop, iridescent rays are 

 emitted by the decomposed light. 



Several examples are also introduced, in which a high order of 

 lenticular correction beautifully discovers structure hidden, according 

 to Dr. Carpenter, F.R.S., from the great bulk of observers. 



As the paper deals so often with magnitudes very much less than 

 the 1-100, 000th of an inch, a method is introduced of readily 

 estimating roughly such magnitudes between the 1-80, 000th and the 

 l-500,000th of an inch, by means of a micrometer gauge. The 

 writer has been emboldened to grapple with these difficult minutiae, in 

 consequence of the sharp and clear definition he has attained of spider 

 lines miniatured down to the fourteenth part of a hundred-thousandth 

 of an inch. The eye, accustomed to contemplate this subtlety of 

 form, readily appreciates the one-fourth or sixth of this size, i.e., 

 l-400,000th or l-600,000th. 



The writer has also ventured to bring before the notice of the Royal 

 Society, a new test for the microscope, displaying bright lines of 

 uniform thickness less than the 1-1 00, 000th, and sharp black lines of 

 much less tenuity than those given by Robert's celebrated lines ruled 

 on glass, and incomparably more easy of illustration. 



VOL. XXIX. N 



