We wished to cover a large area, 6mm in diameter, in order to be able to record the full 3D structure of early embryos (e.g. of mice) and see a wide expanse of the surface of the brain. We also needed a large gap (3mm) between the specimen and the lens, for ease of handling the specimen.

These requirements meant that the lens had to be very large. It is far too large to fit on an ordinary microscope, and we have to focus by moving the specimen, not the lens. Because we have disobeyed one of the basic principles of microscope design by having such large glass elements, we have to make the elements very accurately and position them with an accuracy of a few microns inside the metal tubes.

The elements for the prototype lens were manufactured and mounted by ICOS Ltd in London and the central reflector casing was made in the mechanical workshops of the MRC Laboratory of Molecular Biology, Cambridge.