2-2 



The Rev. T. R. Robinson on the Relation between the Temperature of 



V must be a number less than unity, and that e^<^' " ' must be so large that 

 the last term of the equation may be neglected, whence 



e-T^i, 



''^x^^-^ 



(9) 



When V is known 6 is easily found ; for, entering the table with T as argu- 

 ment, we get a first approximation, with which as argument the true value is 

 obtained. 



A still more accurate mode of obtaining this quantity is by observing the 

 value of X at which the wire assumes a given temperature. That which I se- 

 lected is the point at which platinum begins to be visible in total darkness. It 

 is imknown ; but by varying the current, so as to have dififerent lengths of x, 

 we may equate the values of yin (5). I measured a; by a screen moved by a 

 rack, which was lowered till it cut off all light. Its pinion was moved by a gra- 

 duated head, and I found the measures very consistent. 

 With the wire ^ I obtained 



T= 539.3 ; C= 2.213 ; A = 150.9 ; no light visible. 

 r = 772.9 ; X = 1'.29 ; C = 2.375 ; A = 180.9 ; 

 7"'= 850.5 ; X = 1'.03 ; C = 2.516 ; A = 190.9. 



From these I find v = 0.3 nearly, and with this value 

 e = 710.9 ; -J- = 26.2. 



e' = 911.5 ; -^ = 28.5 ; y' = 739.7. 



e"= 995.5 ; ^^28.6 ; y" ^ 733.2. 



R being =: \/ A x C. 



The hght of the wire is like Herschel's lavender ray, and is, perhaps, rather 

 a phosphorescence than a true ignition. 



For the wire -j4^ the same process gives v — 0.4 nearly, as might be ex- 

 pected from its being proportional to \^m ; the temperature is also some- 



