Maxwell's "Electricity and Magnetism" was published in 1873. Shortly afterwards there were placed in his hands, by the Duke of Devonshire, the Cavendish Manuscripts on Electricity, already alluded to. To these he devoted much of his spare time for several years, and many of Cavendish's experiments were repeated in the laboratory by Maxwell himself, or under his direction by his students. The introductory matter and notes embodied in "The Electrical Researches of the Honourable Henry Cavendish, F.R.S.," afford sufficient evidence of the amount of labour he expended over this work. The volume was published only a few weeks before his death. Another of Maxwell's publications, which, as a text-book, is unique and beyond praise, is the little book on "Matter and Motion," published by the S.P.C.K.
In 1878 Maxwell, at the request of the Vice-Chancellor, delivered the Rede Lecture in the Senate-House. His subject was the telephone, which was just then absorbing a considerable amount of public attention. This was the last lecture which he ever gave to a large public audience.
It was during his tenure of the Cambridge chair that one of the cottages on the Glenlair estate was struck by lightning. The discharge passed down the damp soot and blew out several stones from the base of the chimney, apparently making its way to some water in a ditch a few yards distant. The cottage was built on a granite rock, and this event set Maxwell thinking about the best way to protect, from lightning, buildings which are erected on granite or other non-conducting foundations. He decided that the proper course was to place a strip of metal upon the ground all round the building, to carry another strip along the ridge-stay, from which one or more pointed rods should project upwards, and to unite this strip with that upon the ground by copper strips passing down each corner of the building, which is thus, as it were, enclosed in a metal cage.
After a brief illness, Maxwell passed away on November 5, 1879. His intellect and memory remained perfect to the last, and his love of fun scarcely diminished. During his illness he would frequently repeat hymns, especially some of George Herbert's, and Richard Baxter's hymn beginning
"Lord, it belongs not to my care."
"No man ever met his death more consciously or more calmly."
It has been stated that Thomas Young propounded a theory of colour-vision which assumes that there exist three separate colour-sensations, corresponding to red, green, and violet, each having its own special organs, the excitement of which causes the perception of the corresponding colour, other colours being due to the excitement of two or more of these simple sensations in different proportions. Maxwell adopted blue instead of violet for the third sensation, and showed that if a particular red, green, and blue were selected and placed at the angular points of an equilateral triangle, the colours formed by mixing them being arranged as in Young's diagram, all the shades of the spectrum would be ranged along the sides of this triangle, the centre being neutral grey. For the mixing of coloured lights, he at first employed the colour-top, but, instead of painting circles with coloured sectors, the angles of which could not be changed, he used circular discs of coloured paper slit along one radius. Any number of such discs can be combined so that each shows a sector at the top, and the angle of each sector can be varied at will by sliding the corresponding disc between the others. Maxwell used discs of two different sizes, the small discs being placed above the larger on the same pivot, so that one set formed a central circle, and the other set a ring surrounding it. He found that, with discs of five different colours, of which one might be white and another black, it was always possible to combine them so that the inner circle and the outer ring exactly matched. From this he showed that there could be only three conditions to be satisfied in the eye, for two conditions were necessitated by the nature of the top, since the smaller sectors must exactly fill the circle and so must the larger. Maxwell's experiments, therefore, confirmed, in general, Young's theory. They showed, however, that the relative delicacy of the several colour-sensations is different in different eyes, for the arrangement which produced an exact match in the case of one observer, had to be modified for another; but this difference of delicacy proved to be very conspicuous in colour-blind persons, for in most of the cases of colour-blindness examined by Maxwell the red sensation was completely absent, so that only two conditions were required by colour-blind eyes, and a match could therefore always be made in such cases with four discs only. Holmgren has since discovered cases of colour-blindness in which the violet sensation is absent. He agrees with Young in making the third sensation correspond to violet rather than blue. Maxwell explained the fact that persons colour-blind to the red divide colours into blues and yellows by the consideration that, although yellow is a complex sensation corresponding to a mixture of red and green, yet in nature yellow tints are so much brighter than greens that they excite the green sensation more than green objects themselves can do, and hence greens and yellows are called yellow by such colour-blind persons, though their perception of yellow is really the same as perception of green by normal eyes. Later on, by a combination of adjustable slits, prisms, and lenses arranged in a "colour-box," Maxwell succeeded in mixing, in any desired proportions, the light from any three portions of the spectrum, so that he could deal with pure spectral colours instead of the complex combinations of differently coloured lights afforded by coloured papers. From these experiments it appears that no ray of the solar spectrum can affect one colour-sensation alone, so that there are no colours in nature so pure as to correspond to the pure simple sensations, and the colours occupying the angular points of Maxwell's diagram affect all three colour-sensations, though they influence two of them to a much smaller extent than the third. A particular colour in the spectrum corresponds to light which, according to the undulatory theory, physically consists of waves all of the same period, but it may affect all three of the colour-sensations of a normal eye, though in different proportions. Thus, yellow light of a given wave-length affects the red and green sensations considerably and the blue (or violet) slightly, and the same effect may be produced by various mixtures of red or orange and green. For his researches on the perception of colour, the Royal Society awarded to Clerk Maxwell the Rumford Medal in 1860.
Another optical contrivance of Maxwell's was a wheel of life, in which the usual slits were replaced by concave lenses of such focal length that the picture on the opposite side of the cylinder appeared, when seen through a lens, at the centre, and thus remained apparently fixed in position while the cylinder revolved. The same result has since been secured by a different contrivance in the praxinoscope.
Another ingenious optical apparatus was a real-image stereoscope, in which two lenses were placed side by side at a distance apart equal to half the distance between the pictures on the stereoscopic slide. These lenses were placed in front of the pictures at a distance equal to twice their focal length. The real images of the two pictures were then superposed in front of the lenses at the same distance from them as the pictures, and these combined images were looked at through a large convex lens.
The great difference in the sensibility to different colours of the eyes of dark and fair persons when the light fell upon the fovea centralis, led Maxwell to the discovery of the extreme want of sensibility of this portion of the retina to blue light. This he made manifest by looking through a bottle containing solution of chrome alum, when the central portion of the field of view appears of a light red colour for the first second or two.