334 JAMES CLERK MAXWELL. [CHAP. XI. 



rate eyes. But if you can get observations to be con- 

 sistent to the 3 rd * place of decimals, glory therein, and let me 

 know what the human eye can do. 



Donkin gave me tea in Oxford, July 1, 1860. 



I find that my belief in the reality of State affairs is no 

 greater in London than in Aberdeen, though I can see the clock 

 at Westminster on a clear day. If I went and saw the parks 

 of artillery at Woolwich, and the Consols going up and down 

 in the city, and the Tuscarora and Mr. Mason. I would 

 know what like they were, but otherwise a printed statement 

 is more easily appropriated than experience is acquired by 

 being near where things are being transacted. 



I am getting a large box made for mixture of colours. 

 A beam of sunlight is to be divided into colours by a prism, 

 certain colours selected by a screen with slits. These 

 gathered by a lens, and restored to the form of a beam by 

 another prism, and then viewed by the eye directly. I 

 expect great difficulties in getting everything right adjusted, 

 but when that is done I shall be able to vary the intensity 

 of the colours to a great extent, and to have them far purer 

 than by any arrangement in which white light is allowed to 

 fall on the final prism. 



I am also planning an instrument . for measuring 

 electrical effects through different media, and comparing 

 those media with air. A and B are two equal metal discs, 

 capable of motion towards each other by fine screws ; D is 

 a metal disc suspended between them by a spring. C ; E is 

 a piece of glass, sulphur, vulcanite, gutta-percha, etc. A 

 and B are then connected with a source of+ electricity, 

 and D with - electricity. If everything was symmetrical, D 

 would be attracted both ways, and would be in unstable 

 equilibrium, but this is rendered stable by the elasticity of 

 the spring C. To find the effect of the plate E, you work 

 A further or nearer till there is no motion of D consequent 

 on electrification. Then the plate of air between A and D 

 is electrically equivalent to the two plates of air and one of 

 glass (say) between D and B, whence we deduce the coeff*- 

 for E. 



