136 PROCEEDINGS OF THE AMERICAN ACADEMY 



Having obtained a sufficient number of sets, as perfect as may be, 

 we have next to effect a comparison between the deflections produced 

 by the moon, and the deflections brought about by the sun under simi- 

 lar circumstances of observation, — such as altitude, height of barom- 

 eter, etc. The heat of the sun's rays must be reduced in some 

 manner in order to bring them within ihe range of the same instru- 

 ment employed in measuring the feeble radiation of the moon. Both 

 Lord Rosse and Langley accomplished the reduction by passing the 

 sun's rays thi'ough a small opening, and placing the heat-measuring 

 appliance in the diverging beam. 



The enormous differences that appear in the results of observers 

 who have measured the light of the moon, and compared it with sun- 

 light, put us upon our guard against sources of error, and impressed 

 upon us the necessity of going over the ground by independent 

 methods. Let us consider the probable sources of error in the above 

 mentioned method of comparison : — 



1 . Errors of observation. These are small, and can be reduced to 

 any extent by repeating the observations. 



2. Absorption by the mirror, used in condensing the lunar rays. 

 The mirror is always kept perfect by frequent silvering, and a flat 

 mirror, silvered by the same process, is employed in reflecting the 

 sun's rays to the small opening. 



3. The plate of metal in which is the perforation becomes a 

 source of heat when the sun's rays are thrown upon it, and would 

 increase the heat indication. This effect is avoided by placing the 

 thermograph so far from the metal plate as to render its influence 

 practically imperceptible. 



4. Effect of diffraction at the opening. Unknown. 



As an example of the results obtained by this method, let us take 

 the observations of April 25 and 26, 1888. 



On the night of April 25, the deflection produced by the moon at an 

 altitude of 42° was 184.8 scale divisions. 



We have also : 



Semidiaraeter of condensing mirror . . 95 mm. 

 Focus " " . . 1000 mm. 



Semidiameter of moon, augmented . . 17'. 



Hence we have, for the concentration of the moon's rays, 



^ 369.1. 



(1000 X sin 17')- 



