The Problems with Lenses, and the 19th-century Solution

One of the central themes of the history of the microscope is the history of lens development. Until 1830 two problems hindered lens manufacture: spherical aberration and chromatic aberration. Around 1830, in collaboration with Joseph Jackson Lister, William Tulley made one of the first microscopes that corrected for both chromatic and spherical aberration.

Two types of aberration

Spherical aberration results in a partially blurred image, and is caused by light passing through different areas of the lens. This results in different parts of the image focusing at a different distance between the lens and the eye of the observer. If this is left uncorrected, there is no way to focus the whole of the image at the same time. Towards the end of the 18th century this problem was solved by using a certain combination of lenses, one correcting the aberration of the other.

A similar solution was found, in 1830, for chromatic aberration. This problem results from the fact that the wavelengths of different colours refract to differing degrees, so blue will focus closer to the lens than red The consequence of this is a halo of colours around any object you look at. For example, in Image 1, slight chromatic aberration can be seen around the head of the flea (top right). In this case the aberration is not sufficient to obscure the image, and the flea is clearly visible in other areas, for example its legs (bottom left).

A 'corrected' microscope

The description of the first lenses that were free from both of these aberrations was published in 1830 by Joseph Jackson Lister (1786-1869). Although achromatically corrected lenses had been made before Lister's work, he was the first to describe a full achromatically and spherically corrected optical system for the compound microscope. Lister was not an instrument maker, so he worked with craftsmen such as William Tulley to produce the lenses (Image 2). In the first decades of the 19th century, many attempts had been made to make achromatically corrected lenses for the microscope. Tulley himself had attempted and failed to make a lens that was corrected for both aberrations in 1807.

It was easier to deal with abberation for telescopers lenses, and as solutions were already known before the time microscope lenses were corrected. Telescope lenses are larger, and so it is far easier to grind them for correction. Alternatively mirrors could be used, and reflecting telescopes were popular. Reflecting microscopes were made, but like the lenses, small mirrors were difficult to construct accurately.

Whilst the removal of chromatic and spherical aberration was certainly an important step forward in the development of the microscope, it is not clear how directly it affected the history of scientific discovery.