Prof. S. P. Langley on Energy and Vision. 7 



The observer, in a room completely darkened, except for 

 the minute light diffused from the particles in the reflected 

 beam, and himself shielded even from the feeble light diffused 

 from the surfaces of the lens, the prism, and the mirror, by 

 the thick black curtain shown on the plan, waited until his eye 

 had become quite sensitive before making the readings. An 

 assistant outside the curtain set the circle by the aid of a dark 

 lantern, and adjusted the siderostat from time to time so as to 

 keep the light exactly on the centre of the lens and prism 

 face. The passage of the slightest wisp of cirrus cloud was 

 noted and the observer warned. 



Althouo-h the lio-ht divero-es from slit 2 and not from a 



^ O c5 O 



point, the '' cone of rays " above referred to, is, as regards the 



Figure 2. 

 ScEEEN, half full size. 



object and limits of our experiments and the Hmiting positions 

 of the screen, so nearly coincident with a geometrical cone, 

 that, as the slider is carried away from the slit, the light may 

 be treated as diminishing proportionally to the inverse square 



