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



micrometer. The reason for not forming a direct image with a lens 

 was the varying transparency of glass for radiation at different tem- 

 peratures ; the mirrors also enabled us to " dilute " the heat con- 

 siderably, and so obtain convenient direct deflections on the radio- 

 micrometer scale. 



The sketch (fig. 6) shows approximately the shape of the image 

 formed, on a scale about two-thirds full size. 



FIG. 6. Image of the carbons from a tracing. A represents the size of the 

 aperture by which radiation reached the receiving- surf ace. 



The mirror, M 2 , being provided with adjusting screws, it was easy 

 to bring any part of the image, either of the carbons or the pale 

 violet glow of the arc itself, on to the small aperture, the deflection 

 on the scale of the radio-micrometer then giving readings propor- 

 tional to the radiation from the chosen point. 



Magnified to this extent, however, the arc was never steady enough 

 to allow a detailed " mapping- out " of the carbon surfaces with 

 regard to temperature. Even when the light is apparently steady to 

 the eye, the violet arc itself often shifts its position, while the - pole 

 continually alters in shape from the carbon deposited on it, which 

 causes a bulbous excrescence, somewhat as shown in fig. 7, to form 

 gradually. 



When this is the case, the arc naturally strikes across from some 

 such position as A to B ; B then becomes, as might be expected, 

 much hotter than any other part of the pole. 



As an example of the kind of difference existing between the two 

 poles, the following figures may be given ; they correspond to the 

 hottest obtainable point in the crater of the 4-pole, and to the hottest 

 point on the pole, before any excrecence ha^s had time to grow. 



