571 



When first we make the beam fall over the edges of the plate 

 that is placed immediately before the Ihermopile, we see on the 

 thermopile, besides the illiiminaled plate, 2 lines of light, with the 

 shadow of the plate between them. 



We now slide the doors towards each other, till tlie two lines of 

 light have just disappeared from the thermopile, which can be very 

 sharply observed. Accordingly the bea/n of light now falls exclusively 

 on the plate before the pile. 



When the thermopile is used the width is made equal to about 

 one mm. in a corresponding way, only instead of screen doors use 

 is made here of a cylinder lens. 



With regard to the accuracy we observe that it follows from the 

 table that 2 beams of equal intensity, but of the widths 1 and 0,5 

 mm., both adjusted at the maximum, will show about a ditference 

 of deviation of 2.5 "/o- It follows from this that if we perhaps make 

 the widths equal to about 0.1 mm. in this way, no great errors will 

 henceforth be made with this either. 



The question whether the above given formulae may really be 

 applied here, is fully entered into in my Thesis for the Doctorate. 

 Nor will the constructions be discussed here, which were executed to 

 my great satisfaction by the chief instrumentmaker of tlie laboratory, 

 Mr. JoH. DE Zwaan. 



As a further elucidation of the investigation we shall proceed to 

 give a numerical example. 



