On Thermal Radiation in Absolute Measure. 543 



from each other by hair sharp ed^es, which disappear com- 

 pletely, so that the whole field ABCD looks like a uniform, 

 diffusely illuminated surface, But as soon as one introduces 

 thin glass plates at proper points in front of the two illumi- 

 nated sides of the cube the " principle of contrast" is added 

 to the " principle of equality.-" The fields I. and II. are of 

 uniform intensity, in consequence of which their separating- 

 edge pq disappears; the fields III. and IV. have another 

 intensity, but the difference between I. and IV. being the 

 same as the difference between II. and III., they show equal 

 contrast. 



In the treatise * entitled " Die photometrischen Apparate 

 der Reichsanstalt fur den technischen Gebrauch/' our photo- 

 meter supplied with this " Gleichheits and Contrast-Wurfel " 

 is described and illustrated in the form in which it is con- 

 structed for technical use according to our designs by the 

 Berlin firm, Franz Schmidt and Haensch. Whoever com- 

 pares this photometer with Swan's prism-photometer will with 

 an impartial judgment hardly agree with Prof. Knott when 

 he exclaims, with reference to our photometer. " Let us 

 now, who know its value, not forget that it is i Swan's ' 

 photometer/' 



LV. Thermal Radiation in Absolute Measure. By J. T. 

 Bottomley, ALA., D.Sc. F.R.S., and J. C. Beattie, 

 D.Sc, F.R.S.E.f 



fl^HE experiments J described in the following paper form a 

 JL continuation of researches on thermal radiation by one 

 of the present authors, the results of which have been com- 

 municated to the Royal Society from time to time since 1884 § . 

 The main object of the present experiments was to push 

 forward the inquiry as to the amount, and the relative quality, 



* Photoni. Untersuchungen, iv. : Zeitschrift f. Instrkde, xii. pp. 41-50 

 (1892). 



f Communicated by Lord Kelvin ; having- been read before the Royal 

 Society, Feb. 1, 1900.* 



X The experimental results of the paper were obtained two years ago. 

 Various circumstances have prevented earlier publication ; and it was 

 originally intended to carry the investigation further before publishing. 

 Want of opportunity, however, makes this difficult for the present ; and 

 we therefore deem it advisable to put our results on record just now, as 

 they stand. The present investigation, as well as the former work referred 

 to in the text above, has been assisted by grants from the Government 

 Grant Fund. 



§ " On Thermal Radiation in Absolute Measure.'' J. T. Bottomley, 

 Roy. Soc. Proc. and Phil. Trans. L884-1893, 



