17 
boundary bands. This total evanishment of the Podura 
markings is, perhaps, one of the most extraordinary pheno- 
mena of its kind ever observed.! It will be seen that the 
more acutely the substrata of ribbing or beading cross the 
upper, the longer are the black lines of interference, and the 
more prolongated are the false spectra or spurious spines 
(fig. 7). 
§ III. On the possibility of searching the Axis for the most 
favorable Position of the Conjugate Focus to form an 
Image with a Minimum Aberration. 
I have already remarked that objectives are usually con- 
structed to give the most beautiful (I might say finished) 
image for the eye-lens at a final distance of eight or nine 
inches, beyond and within which the performance is not 
guaranteed. 
But it appears, from a discussion of the equations of aberra- 
tion of a series of lenses separated or in contact, that the 
conditions of aplanatism may be satisfied by an indefinite 
number of solutions, especially, ceteris paribus, by fortu- 
nate selections of the values of wand v, the distances of the 
object and final image from the front lens. A search, there- 
fore, along the axis of a given instrument may be rewarded 
with the discovery of an aplanatic focal point even better 
than the conventional distance. And, supposing that point 
can be found, the residuary aberration may in many cases be 
further ameliorated by introducing a new compensation of 
position and separation. 
Together with these acquisitions it is desirable to combine, 
as well for the immersion as ordinary objectives— 
(1) A more convenient distance of the front lens from the 
covering glass. 
(2) To increase depth of perspective. 
(3) To use low power and least aberrating lenses for the 
eye-glasses. 
(4) To exalt magnifying power. 
I venture to hope that the aplanatic searcher used under 
the conditions to be specified will be found to accomplish some, 
if not all, these desiderata. 
__1 First observed with 2,th immersion lens, aplanatic searcher, and oblique 
illumination at right angles to markings, July, 1868. 
(Zo be continued.) 
VOL. XI.—NEW SER. B 
