1879O Proceedings of Societies . 577 
angular aperture. 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 re- 
fracting 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 oblitera- 
tions. And that, in consequence, innumerable minute details 
are concealed or destroyed till the aperture is sufficiently re- 
duced. That minute refracting bodies obey the laws of their 
refrangibilities 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, translucency, 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 i-ioo,oooth of an inch, a method is intro- 
duced of readily estimating roughly such magnitudes between 
the i-8o,oooth and the 1-500, oooth 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 ., 1 -400,000th 
or 1-600, oooth. The writer has also brought before the notice of 
the Royal Society a new test for the microscope, displaying 
bright lines of uniform thickness less than the 1-100, oooth, and 
sharp black lines of much less tenuity than those given by 
Nobert’s celebrated lines ruled on glass, and incomparably more 
easy of illustration. The employment of various fluids for 
immersion lenses is carefully considered ; and the singular pro- 
perty of castor oil discovered by the writer is referred to. But, 
as the author’s paper on “ A Searcher for Aplanatic Images” 
was inserted in the “ Transactions,” he now introduces a new 
form which offers some advantages : by its extended traverse, 
by its simplicity and economy of light with increase of magnifying 
power. Finally, some examples are given of producing trans- 
cendentdefinition in cases found hopeless by a numerous body 
of observers. The means also of its attainment are minutely 
described. 
“ Preliminary Note on a new Tide-predidtor,” by E. Roberts, 
F.R.A.S. The Indian Survey Department having under- 
VOL. .IX, N.S.) 2 0 
