James Clerk Maxwell is particularly acknowledged among scientists for combining the theories of electricity and magnetism into electromagnetism, described by Maxwell’s equations. However, his research was very wide ranging, including an interest in how our eyes see colour.

Maxwell (1831-1879) was born in Edinburgh, and in his comparatively short life became one of the world’s greatest physicists. He studied at the Universities of Edinburgh and Cambridge, before holding professorships at Marischal College in Aberdeen, King’s College London, and Cambridge.

An interest in colour

James Clerk Maxwell’s interest in colour vision started when he was a young man. He was introduced to the topic when he was 18, by one of his professors at the University of Edinburgh, James Forbes. Maxwell went on to study at the University of Cambridge, and while there carried out research into colour vision. He was particulalry interested in how people see mixtures of colours. If two colours are mixed, do we all see the same result?

To test this he used a colour top. He had been introduced to these by Forbes and was photographed holding this first top as a young man. This top itself survives in the collections of the Cavendish Laboratory in Cambridge.

Maxwell's tops

Maxwell soon invented a smaller top, which was probably easier to spin fast. It was made for him by the optical instrument maker Bryson of Edinburgh. It consisted of a metal disk with a scale around its edge which was divided into 100. It had paper disks of two sizes painted with artists pigments (named on their backs) and an axle which held the whole together and allowed it to spin.

When the top is rapidly spun, faster than the eye can follow, the colours seem to blur together. Two colours are seen, one in the outer ring, and one in the centre. If the disks have been correctly chosen, and the proportions of different colours carefully adjusted, both the outer ring and the inner circle can appear the same colour when the top is spun. The spinning discs mixed light which gives a different result to mixing paint.

Maxwell carefully recorded the different colours and proportions necessary to get the inner and outer colours to match. He found that the proportions of colours which were seen as the best match depended slightly on the observer. A few people who were colour blind gave very different answers.

The results also changed according to the light source. Most of his readings were taken in sunlight, but different results were obtained looking at the top under Edinburgh gaslight. The exact colour of gaslight varied because of differences in local supply and contaminants in the gas. The burner design would have also played a part.

Being one of Maxwell’s observers must have been a fairly long and likely tedious task. It required staring repeatedly at the spinning top and describing the difference between the two colours. In 1859 he wrote to his childhood friend, the physicist Peter Guthrie Tait:

Maxwell’s 1860 paper on colour vision featured two main observers ‘J’ and ‘K’. ‘J’ was James Clerk Maxwell himself, and it is clear from contemporary letters that the anonymous ‘K’ was his wife Katherine. They collaborated on experiments both before and after their marriage.

Tops in our collection

Three examples of this top are in the National Museums collection, without their axles. One had belonged to James Forbes, who started Maxwell’s interest in colour vision. Another belonged to Edinburgh University, and the coloured disks of the third were discovered tucked into the drawers of an earlier piece of apparatus, a predecessor of Maxwell’s top made in about 1830.

Another Edinburgh academic very interested in colour blindness was George Wilson, the founding Director of the Industrial Museum of Scotland, now the National Museum of Scotland. He published in 1855 what is acknowledged as the first book on colour blindness, including concern for the safety of coloured signals. (Why, he asks, are signal lights red and green, when those are the colours confused in the most common form of colour blindness?) He also corresponded with Maxwell, and reprinted Maxwell’s research as an appendix to this book.

Experiments with colour photography

In 1855 Maxwell’s research into colour vision led him to suggest how to take a photograph that would appear to be fully coloured, using three black and white slides and three coloured filters.

In 1861 he commissioned Thomas Sutton to take a demonstration photograph of a tartan ribbon which he showed projected onto a screen at King’s College London. This image shouldn’t have worked as well as it did, because the photographic chemicals did not respond to red light. Serendipitously, unseen ultraviolet light also reflected off the red portions of the ribbon and provided the third colour.

The first colour photographs used this method, as suggested in 1855 by James Clerk Maxwell. Three separate images were taken in three different colours and projected onto the same surface, or, as with the peacock feather shown here, viewed stacked up. 

Modern TVs and computer screens use this same technique of mixing light, combining tiny emitters of red, green and blue light into each pixel. Both use this same technique of mixing light, combining tiny emitters of red, green and blue light into each pixel. This is optimised to work with human eyes, and will look very wrong to many animals that have eyes sensitive to different wavelengths from people. For instance, some animals can see infrared or ultraviolet as well as what we call visible light.

Maxwell's further experiments

Also in our collection is a dynamical top Maxwell worked on from 1856. The dynamical top is another form of spinning top which acts as a demonstration gyroscope.

Maxwell initially made wooden examples of the top. He then turned to the instrument makers Smith and Ramage of Aberdeen (where he was then Professor at Marischal College) to make a brass example.

The nuts which stick out of the top can be screwed in or out to exactly adjust its balance on the stand and make it show a variety of motions. A disk with coloured sectors or patterns was mounted on the top, and clarified the motion of the top, which rotates faster than the eye can follow. This research was useful for demonstrating how the Earth spins, as it is not a perfect sphere.

This particular dynamical top was commissioned by Maxwell’s former professor James Forbes of the University of Edinburgh. In 1857 Forbes asked Maxwell to get him ‘a Dynamical Top made for my Class on the pattern of your own’. This appears to have been one of four which Maxwell arranged for Smith and Ramage to make for different universities at this time. They cost £3 3s each, which corresponds to about £1800 worth of skilled craftsmanship in today’s money.

In Maxwell’s obituary in the journal Nature there is a story about the dynamical top. He demonstrated this one night to friends in his room while on a visit to Cambridge and it was still spinning when they left. Early the next morning, Maxwell noticed one of the friends coming towards his room, so he set the top spinning again, and returned to bed, giving the impression it had spun all night.

Maxwell died at the age of 48, and is buried at Parton Kirk, close to where he grew up.