526 ■'- ■ REPORT — 1881. 



2. On some Colour Experiments.^ By Lord Ratleigh, F.B.S. 



3. On a Question in the Theory of Lighting . By Lord Ratleigh, F.B.S. 



It is known that a large part of the radiation from terrestrial sources is non- 

 luminous. Even in the case of the electric arc the obscure radiation amounts, ac- 

 cording to Tyndall, to eight-ninths of the whole, and of the remainder probably no 

 inconsiderable part is to be found in the extreme red rays of feeble limiinosity. 

 For practical purposes this obscure radiation is useless ; and the question forces 

 itself upon us, whether or no there is any necessity, absolutely inherent in the 

 case, for so large a proportion of waste. The following arrangement, not of course 

 proposed as practical, seems to prove that the question should be answered in fjxe 

 negative. 



Conceive a small spherical body of infusible material, to which energy can be 

 communicated by electricity or otherwise, to be siurounded by a concentric re- 

 flecting spherical shell. Under these circumstances no energy can escape ; but if a 

 small hole be pierced in the shell, radiation will pass through it. In virtue of the 

 suppositions which we have made, the emergent beam will be of small angle, and 

 may be completely dealt with at a moderate distance by a prism and lens. Let us 

 suppose then that a spectrum of the hole is formed and received upon a reflecting 

 plate so held at the focus as to return the rays upon the lens and prism. These 

 rays will re-enter the hole, and impinge upon the radiating body, which is thus 

 again as completely isolated as if the shell were unperforated. We have now only 

 to suppose a portion of the focal plate to be cut away in order to have an apparatus 

 from which only one kind of radiation can escape. Whatever energy is communi- 

 cated to the internal body must ultimately undergo transformation into radiation of 

 the selected kind. 



4. On some uses of Faure's Accumulator in connection with Lighting hy 

 Electricity. By Professor Sir William Thomson, M.A., F.B.S. 



The largest use of Faure's accumulator in electric lighting was to allow steam 

 or other motive power, driving dynamos, to work economically all day, or through- 

 out the twenty-four hours where the circumstances were such as to render this 

 economical, and storing up energy to be drawn upon when the light was required. 

 There was also a very valuable use of the accumulator in its application as an 

 adjunct to the dynamo, regulating the light-giving current and storing up an 

 irregular surplus in such a manner that stoppage of the engine would not stop 

 the light, but only reduce it slightly, and that there would always be a good 

 residue of two or three hours' supply of full lighting power, or a supply for eight 

 or ten hours of light for a diminished number of lamps. He showed an automatic 

 instrument which he had designed and constructed to break and make the circuit 

 between the Faure battery and the dynamo, so as automatically to fulfil the con- 

 ditions described in the paper. This instrument also guarded the coils of the 

 dynamo from damage, and the accumulator battery from loss, by the current 

 flowing back, if at any moment the electro-motive force of the dynamo flagged 

 so much as to be overpowered by the battery. 



5. On the Economy of Metal in Conductors of Electricity. 

 By Professor Sir William Thomson, M.A., F.B.S. 



The most economical size of the copper conductor for the electric transmission of 

 energy, whether for the electric light or for the performance of mechanical work, 

 would be found by comparing the annual interest of the money value of the copper 

 with the money value of the energy lost annually in the heat generated in it 

 by the electric current. The money value of a stated amount of energy had not 



' Published in extenso in Nature,'SoY. 17, 1881. 



