Power of Lamp- and Platinum- Black 

 The most satisfactory way of 



169 



lamp. The most satisfactory way of measuring their 

 thickness was to place the surfaces nearly vertical under the 



__ 



Position of Thermopile 



microscope after the conclusion of the experiments and to 

 take the mean thickness of clean edges cut in the lampblack 

 at all parts. The platinum-black surfaces were deposited 

 -according to the recipe of Lummer and Kurlbaum for 

 different lengths of time. 



A galvanometer deflexion of 1 mm. corresponded to a 

 •current of 0*7xl0 -10 ampere; the deflexions were pro- 

 portional to the current. 



The reflective power was determined for the following 

 radiations whose maxima lie in the infra-red : — 



1. That part of the radiation from an incandescent mantle 



which is transmitted by a layer of water 1 cm. thick. 

 The water absorbs wave-lengths larger than 1 /jl *. 

 The maximum of the transmitted radiation lies at 

 about 0'8 /a. 



2. The Heststrahlen from Selenite. A parallel beam 



from a slit illuminated by an incandescent mantle 

 was reflected at the surfaces of three plates of 

 gypsum before being focussed on the thermopile. 

 The maximum lies at S'7 fi f« 



* Paschen, Wied. Ann. lii. p. 209 (1894) and others, 

 f Aschkinass, Ann. d. Phys. i. p. 42 (1000) and others. 



