26 



Messrs. W. E. Wilson and P. L. Gray. On the 



The size of the hole in the screen S, and its distance from the radio- 

 micrometer, then give the apparent area of bright carlbon as seen 

 from the latter. 

 The screen S is made of copper, and is really a flat box (Fig. 2) 



FIG. 2. 



provided with an inlet and an outlet tube, so that a continual stream 

 of water from the ordinary house supply could be kept running 

 through it a precaution necessary from its proximity to the 

 arc. In the experiments, a plentiful stream was kept running 

 through this box, and thence on to the water-jacket round the 

 radiator, the supply being sufficient to prevent any perceptible 

 heating of the screen. 



A small hole cut in the side of the wooden box enabled us, with 

 the aid of mirrors, to use a pencil of the light of the arc for reflection 

 from the mirror of the radio-micrometer, thus obviating the necessity 

 of a lime-light, or other bright source, while an incandescent lamp- 

 filament provided us with an extremely sharp band of light on the 

 scale of the radiator. (A larger and better mirror had been affixed 

 to this since its use in our work on the solar temperature, and this 

 mirror, with an incandescent lamp, gives a band of light with edges 

 so sharp on the " temperature scale," that it could, if necessary, be 

 read to the tenth of a millimetre, which is beyond our ordinary 

 requirements.) 



The theory of the method is very simple ; essentially it is the same 

 as that which applies to the estimation of the effective temperature 

 of the sun, without the complications arising from atmospheric 

 absorption, &c. 



In the case of the sun, we can only hope to find (at least at present) 

 the effective temperature, as we know little of the radiating power of 

 the photospheric substances, but in the case of the carbons of the arc, 



