256 Prof. E. Wiedemann on the 



The influence of these disturbing circumstances may be 

 determined by heating a platinum wire to a definite tempera- 

 ture in an air-bath and measuring the intensity of the light 

 emitted at a definite part of the spectrum, and at the same 

 time determining its resistance. Then the wire is heated by 

 means of a current to the same brightness and its resistance 

 determined again. From the difference in resistance observed 

 in the two cases account can be taken of the complications in 

 question. The experiments should be made with wires of 

 different thickness. 



Comparison of the Amyl-acetate Lamp with Glowing Platinum. 



23. From these determinations we will turn to the definite 

 evaluation of the amyl-acetate lamp in absolute measure. 

 Care must be taken that the platinum wire is linear, but 

 the amyl-acetate flame, on the other hand, flat ; i. e. so that 

 the rays from the former traversing the slit only fill a portion 

 of the objective, whilst those from the latterly entirely fill it 

 so soon as the flame is sufficiently near to the slit, as is the 

 case in our experiments. 



How account is to be taken of these circumstances in their 

 influence on the brightness is explained in the following. 



(a) First, we calculate the quantity of energy reaching 

 unit length of the slit from the platinum wire. 



Let h be the diameter of the diaphragm in the collimator 

 which limits the pencil of rays issuing from it, e its distance 

 from the slit, tj the distance of the wire from the slit, and 8 

 the thickness of the wire. The pencil of rays drawn from the 

 diaphragm through a point of the slit intersects on the surface 

 of the wire an area which, projected on the meridian-plane at 

 right angles to the axis of the collimator, has a breadth 8 and 

 a height 7, as calculated from the proportion 



<y : 7] = n : e, <y= — • 



The quantity h/e occurring here can be calculated as follows. 



At a distance a of 35 centim. from the slit a scale was placed 

 at right angles to the axis of the collimator and to the length 

 of the slit, and a light was moved along the scale until an 

 observer at the telescope announced that the light could no 

 longer be seen. On moving the eye sideways right or left the 

 distance of these two points was 2*2 centim. : then 



h/e=l/a=^- =0-063. 

 ' ' 3d-0 



