The Present Limits of Vision. By Dr. Boyston-Pigott. 187 
as achromatism for illuminating lenses. In giving directions for 
the detection of Podura beading six years ago, particular allusion 
was made to this ; and in the present improved state of object- 
glasses, much freed from spherical aberration, as shown by the more 
recent efforts of Messrs. Powell and Lealand, Wenham, and others, 
I may be permitted to again draw the attention of the Society to 
what may still be considered interesting points, and by no means 
exhausted. I said : * — 
'^The extreme difficulty of defining a minute row of beads 
arises from the uncorrected aberrations confusing their images, by 
which several images overlap and obliterate the form of individual 
beads In my experience, I have found an oblique centrical 
pencil of achromatic cones of light of small aperture (15° or 20°) 
of the greatest practical utility, the obliquity being varied accord- 
ing to the object in view. 
" Another circumstance worthy of note, viz. the position of the 
stigmatic image, or of the distance from the back lens of the object- 
glass, where there is a real focus A search for the real focus 
or best image should not be neglected along the axis of the instru- 
ment." So far in 1869. 
According to my own investigations, there are many objectives 
which refuse to give their best defining powers at the particular 
rigid distance of ten inches. How much easier to alter the length 
of tube than the intrinsic corrections of the glasses ! A telescope 
tube, or one built up of pieces, is one of the simplest of all adjust- 
ments for over or under-correction. I prefer to use my 50th 
objective with half the ordinary length. It gives a longer focus, 
allows a thicker covering glass, and displays a brilliance of detail in 
this way sometimes quite amazing. By shortening or lengthening 
the eye tube, the observer is actually making, perhaps unwittingly, 
a most philosophical search for a real aplanatic focus. Very few 
people know that it is the last back lens which makes the last final 
image within the tube at the locus of the eye-piece. Each lens 
makes its own, which in its turn is taken up by the next, and so 
on. Some lenses have one, some two aplanatic foci — i. e , at certain 
points only in the tube. Find them by all means, if you can, by 
thus searching the axis. Perhaps you will get a magnificent effect 
at one length, impossible with another. The screw-collar does a 
good deal, but not all that is required to open out latent beauties 
perhaps hitherto unseen. 
Another point bearing on the subject of this paper is the use of 
the searcher for aplanatic images. A lengthening or shortening 
tube is one of the best of searchers. Combined with this is the 
subtle effect of moving to and fro lenses between the eye-piece 
and objective. 
* 'Moulhly Mic. Jour.' Dec. 1, 1869= Sent to Society, May 21, 18G9, 
