High 'Temperatures by the Method of Colour Identity. 59 



the Wien equation. Curve C is a curve for a grey body 

 with a certain emissivity at the same true temperature. If 

 the temperature of this grey body be measured with an 

 optical pyrometer, in the green (X = 0*547 /jl), it will be given 

 a value of 1578° C. This particular grey body curve has 



Fig. 5. 



50C3 



700 

 |> 600 

 "^ 500 

 •5 400 



300 

 200 

 100 







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046 048 0'50 



0-52 0-54 056 058 

 Wave Length (in^'s). 



060 062 064 066 068 



Energy intensity curves in the visible spectrum for : — 



A. Black body at 1750° C. 



B. „ „ at 1578° C. 



C. A grey „ at 1750° C. 



D. Platinum under open radiation conditions plotted from values 



given by Waidner and Burgess. 



been taken in order that it may be comparable with Waidner 

 and Burgess's platinum curve, in which the black body tem- 

 perature of the platinum melting-point at \ = 0547 fi is 

 given as 1578° C. Curve B is the true black body curve 

 for a temperature of 1578° 0., and has been drawn in order 

 to show the difference between the black body and grey body 

 power curves referred to the same intensity of radiation in 

 the green. 



The ordinates are relatively so small at the shorter wave- 

 lengths that the crossing of curves B and C at X = 0*547^ 

 cannot be distinguished. They do, however, actually cross. 

 It is well to notice from curves B and C what a difference in 

 light distribution at different wave-lengths exists between 



