THE SHIELD MICROSCOPE THEORY

Spherical glass beads act as lenses with magnifications up to about 300x

The end of a 1.5mm diameter glass filament, rotated in a Bunsen flame, will form a molten sphere of glass (yes, even molten glass has surface tension, which draws it into a sphere if sufficiently small).  For soda glass with a refractive index of 1.5 the magnifying power L of a glass sphere diameter D mm is given by L=333 / D and the focal length from the centre of the sphere by 0.75 x D mm. Subtracting half the diameter from the focal distance gives the distance from the surface of the lens to the focal plane. This is summarised thus

Diameter mm

Mag X

Focal plane from surface of lens mm

5

67

1.25

4

83

1

3

111

0.75

2

166

0.5

1

333

0.25

The thickness of a conventional glass cover slip is 1.13mm, thus it can be seen that anything smaller than a 5mm 67x lens could not be focused on the specimen. Shield’s acetate slides have a cover slip of 0.52mm however, with about a 0.35mm gap to the lens surface, enabling a 3mm lens at 111x magnification to be used with the specimen in perfect fixed focus.

Below 1mm in diameter is not really practicable to make; or indeed use, as the focus is so near the surface of the glass - with a different material such as diamond the refractive index is such that the focal plane would be inside the lens.

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This is the really clever innovation, doing away with the complexities of focusing. By careful choice of the thickness of the base plate, the diameter of the hole and the thickness of the cover slip in combination the focal plane can be centred on the specimen.