0. Barus — Coronas of the Fog Layer. 



565 



virtue of the screw extension adjustment s and s', are attached 

 parallel to each other. The lower margin of M is somewhat 

 above the upper margin of M'. Hence the observer on the 

 left of the apparatus (in front) sees the direct rays AA' from 

 the source as well of the reflected rays B M M'B'. By prop- 

 erly adjusting the angle a, the small round distant source of 

 white light and its image in the mirrors (A' and B' respec- 

 tively) may be made to coincide at the upper edge of 21' . In 

 such a case the corona clue to the direct ra} r s produced hj a 

 single thickness, <^=15 - 5 cm of fog layer, should exactly coincide 

 with, i. e. be the complement of, the coronas clue to the reflected 

 rays and produced by a triple thickness ^'=46 - 5 cm of fog layer, 

 if the variation of thickness in question is without effect. Other- 

 wise the coronas should be dislocated at the margin of the 



mirrors. 



3. Results and Summary. — These experiments were carried 

 through in regular series, both for the dust nuclei of ordinary 

 air as well as for an artificial nucleatioii due to phosphorus. 

 The exhaustions were made systematically, every two minutes. 

 The coronas due both, to the A rays (direct) and the B rays 

 (reflected) were read off as quickly as possible, after which 

 filtered air was introduced to dispel the coronas by evaporating 

 the fog particles. In this way about twenty, coronas were suc- 

 cessively compared, from the largest easily observable having 

 an aperture of about 34°, to the small ones of vanishing size, 

 the nucleatioii ranging from about 10 6 to zero (particles per 



Am. Jour. Sci.— Fourth Series, Vol. XXXI, No. 186.— June, 1911. 



