There are, at present, three kinds of apparatus for the production of a picture of a view subtending 180° on a flat plate or film-Wood's fish eye camera, Bond's hemispherical lens and Hill's convexo-concave lens. To the best of our knowledge, a spherical mirror has not been used in practice for such a purpose.
So, it may be of value to give a short description of the apparatus and to show some photographs taken with it.
A convex spherical mirror with a large aperture is arranged to face vertically upwards, and an ordinary photographic camera is placed at some distance above and looking into the mirror to photograph the image of clouds appearing in the mirror. We used a silvered round glass bottle as a spherical mirror. Of course, the perfect sphericity of the mirror cannot be expected from such an ordinary bottle. The surfaces of some glass bottles used by us are, however, nearly spherical. For example, one of them has, excluding the portion near the neck of the bottle, the mean radius of 11.62±0.15cm.
The defect of this, mirror is, however, that some small flaws usually exist somewhere on the surfaces. The ideal spherical mirror with a large aperture is most probably to be obtained by silvering the surfaces of a large condenser. This is, however, beyond the easy reach of the amateur photographer. Now let us study how large the aperture of a mirror is in order to photograph the whole sky.
Denote the angle of the field of view of a photographic lens by ψ, the aperture of a mirror by φ, and the radius of the mirror by γ. The ray coming from the horizon is incident and reflected on
A and
A' on the mirror and reaches
B at which point the lens is placed. (see Fig. 1. page 239).
There is a relation between, ψ and φ. ψ=π-2φ_??_(1) If the distance of
B from
C which is the pole of the mirror is denoted by
d, three parameters
d,
r and ψ are connected by an equation of In our case
d=100cm, , ψ=30°. From (1) and (2) we obtain φ=75°,
r=48.4cm.
Therefore, the largest image of clouds is obtainable by using a spherical mirror of radius of only 48cm with the above mentioned camera.
Some photographs taken by this method are shown in Fig. 2, 3, 4, 5. It is to be noted that the surrounding view below the horizon appears in those pictures. It is wonderful that though the pictures are distorted except near the center, they are distinct even near the horizon in spite of the spherical aberration due to the large aperture of the mirror.
The drawback of this method is that the image of the camera appears in the center of the picture. The damage coming from such an obstacle can however be lessened, if a mirror of large radius is chosen and the camera is placed at a considerable distance from the mirror, since, in this way, the image of the camera become smaller. This becomes evident, if Fig. 3 is compared with Fig. 4, where the former is taken with a large mirror, while Fig. 4 with a small one
(1).
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