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VI . — On Bull's-eyes for the Microscope. 
By E. M. Nelson, F.R.M.S. 
( [Read 18 th March , 1891.) 
Having lately been investigating the optical principles involved in 
the construction of lantern condensers, I have found some points 
which are applicable to, and of service in, the cause of microscopy. 
With regard to the lantern, my aim in the first instance was to 
construct a condenser which could be used either with lime-light 
or with a mineral oil lamp. I designed a triple, consisting of two 
menisci and a plano-convex of crown, the front lens being removable, 
so that the two remaining lenses formed a double condenser for use 
with mineral oil. 
The triple, when tried with lime-light, gave several important 
results : — 
1st. The light secured was about double that given by ordinary 
forms. 
2nd. The definition on the screen was undoubtedly improved. 
This latter result might have been expected from the former, because 
of the increase of light that was sent through the darker portion of 
the picture, but there was also a marked improvement in what is 
known as the high lights, and this certainly I did not expect. The 
conclusion that forced itself upon me was that the case was analogous 
to that of the Microscope — viz. a more perfectly constructed con- 
denser gave a more perfect image. Although my triple greatly 
reduced the spherical aberration there was still a considerable amount 
left, owing to the conditions imposed by having the two lenses suited 
for a mineral oil illuminant. Seeing, however, so much improvement 
in the definition, I have since computed a quadruple condenser of 
minimum aberration, which will, I feel confident, yield still better 
results. 
It was this improvement in the definition which led me to turn 
my attention to Microscope bull’s-eyes. These I found were con- 
structed on principles of maximum aberration, or rather on no 
principles at all. When, some years ago, I adapted the Herschel 
doublet to the Microscope bull’s-eye, thinking that the form was 
well known, I left it entirely in the hands of the optician, but on 
more close examination I now find that the combination supplied has 
no point of resemblance to Herschel’s doublet, of which I have found 
in several old books a description. Many years ago it was discovered, 
by whom I do not know, that light falling on a dense medium 
of refractive index /i bounded by a spherical surface, would, under 
certain conditions, be refracted without aberration to another point. 
The conditions were, that the distance from the point from which the 
