Beck, J. L.

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A New Reflecting Microscope Objective with 
Two Concentric Spherical Mirrors 
J. L Beck 
Department of Scientific and Industrial Research, Physics and 
Engineering Laboratory, Lower Hutt, New Zealand. 
Received 12 February 1969. 
It is well known that a microscope objective, corrected simul-
taneously for spherical aberration, coma, and astigmatism, can be 
July 1969 / Vol. 8, No. 7 / APPLIED OPTICS 1503 
Fig. 1. Scaled drawing of the oil immersion objective described. 
constructed from a concentric pair of spherical mirrors—a large 
concave primary and a small convex secondary. Unfortunately, 
the secondary mirror in these systems obstructs an objectionably 
large central part of each incoming pencil of light. Norris, 
Seeds, and Wilkins1 showed that the central obstruction can be 
reduced while spherical aberration remains corrected with a 
nonconcentric mirror pair, but coma is then no longer corrected. 
Bouwers2 retained concentricity while reducing the effective cen-
tral obstruction with a semirefiecting surface. Steel3 '4 has sug-
gested systems of three concentric mirrors which are as well cor-
rected for aberrations as the concentric mirror pair, but have a 
smaller central obstruction. He appears to have overlooked a 
convenient special case, to which this letter draws attention, in 
which the primary and tertiary mirrors combine to form a con-
tinuous concave spherical mirror surface, as shown in Fig. 1. 
Although the system could have the mirrors facing air, it would 
generally be easier to build with the mirrors immersed in a built-up 
block of glass (or fused quartz, for use in uv light) as suggested in 
the figure. This not only simplifies the support of the reflecting 
surfaces, but also makes possible an increase in numerical aperture 
with oil immersion, or, without oil immersion, correction of field 
curvature with a concave entrance face. In the latter case, both 
longitudinal and transverse chromatic aberration will be absent if 
the entrance and exit faces are concentric, respectively, with the 
axial object and image points. In the case of oil immersion, al-
though longitudinal chromatic aberration will not be present if 
the exit face is concentric with the axial image point, transverse 
chromatic aberration cannot be avoided without an increase in 
complexity of the objective, and is probably better corrected with 
a compensating eyepiece. 
Figure 1 illustrates a particular design for an oil immersion ob-
jective, of this all glass type, that has zonal spherical aberration 
corrected within the Rayleigh limit down to a wavelength of 310 
mm. The design has a numerical aperture of 0.66, a magnifica-
tion of 34.6 × , and a focal length of 2.99 mm when the mirrors are 
in air, while the radius of curvature of the concave pr imary/ 
tertiary mirror is 12.70 mm, that of the convex secondary mirror 
is 2.37 mm, and the short and long conjugates, measured from 
the pole of the primary/tert iary mirror are, respectively, 14.64 
mm and 79.38 mm. The obstruction of the incoming axial pencil 
of light by the secondary mirror is 10% (area). In the all glass 
design, an exit face will have to be polished on the side of the 
cylindrical block of glass to the right depth to allow the light to 
leave the system via the auxiliary plane mirror. This will mean 
that a small segment of the area of the primary /tertiary mirror 
will be lost in the particular design illustrated. 
An objective of the type described is capable of being designed 
with a much higher magnification than that for the particular 
1504 APPLIED OPTICS / Vol. 8, No. 7 / July 1969 
design given here, for sensible use of the large resolving powers 
obtainable with uv light and oil immersion. 
The author wishes to acknowledge the helpful suggestions of 
N. J. Rumsey, Optics Section, Physics and Engineering Labora-
tory. 
References 
1. K. P. Norris, W. E. Seeds and M. H. F . Wilkins, J. Opt. Soc. 
Amer. 41 , 111 (1951). 
2. A. Bouwers, in Proceedings of the London Conference on Optical 
Instruments (Chapman & Hall, London, 1951), pp. 57, 62. 
3. W. H. Steel, Australian J. Sci. Res. 4, 1 (1951). 
4. W. H. Steel, Rev. Opt. 32, 269 (1953). 


A New Reflecting Microscope Objective with Two Concentric Spherical Mirrors

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A New Reflecting Microscope Objective with Two Concentric Spherical Mirrors

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