364 



S. P. Langley — Energy and Vision. 



parable unless they are all taken under the same optical con- 

 ditions, e. g. all with the naked eye or all with glasses of a 

 certain strength. 



In these measurements a magnifying glass of 4*7 cm focus 

 was used by all the observers, and in addition, two who were 

 near-sighted wore spectacles correcting this defect. 



2. 



Screen, one -half size. 



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 center of the lens and. prism 

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

 noted and the observer warned. 



Although the light diverges from slit 2 and not from a 

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



