176 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



cut off the blue and violet rays, yet allow a light to pass 

 whose strongest actinic power is in the green rays. The 

 committee emphatically state that " there is no non-actinic 

 light," only a difference in the rapidity of action, in which 

 latter the violet and the blue have the superiority. Vienna 

 Phot. Corresponclenz, N'ovemher, 1873. 



ACOUSTICS OF LARGE ROOMS. 



A recent study of this subject has been made by Orth, ac- 

 cording to the method of graphic construction as detailed in 

 the treatise of Langhaus, the law of reflection of sound and 

 that of intensity, as affected by varying distance, being main- 

 ly involved. The former consideration is easily introduced 

 in graphic constructions ; the latter necessitates calculations 

 based upon the length of the path of the sound waves. By 

 the employment of 0.01 of a square meter at a distance of 

 ten meters from the origin of sound, as a unit of intensity, 

 Orth was also able to accomplish a graphic representation of 

 intensity. The effect of intersecting waves upon each other 

 he did not regard as demanding practical consideration ; but, 

 on the other hand, the difllision of sound by reflection from 

 rough surfaces proved to be of great importance, as furnish- 

 ing, in some cases, the readiest remedy for acoustic annoy- 

 ances. The only source of acoustical defects in a hall to be 

 considered, according to these investigations, is the reflection 

 of sound waves in such a way that they strike upon the ear 

 of the hearer within a certain interval after the direct waves, 

 and are recognized as an echo or resonance. For graphic 

 determinations this interval of time is converted into differ- 

 ence between the lengths of the paths of the direct and re- 

 flected rays from the origin of sound, and the question of 

 practical acoustics rests essentially on a knowledge of the 

 limits within which this difference exerts a disturbing in- 

 fluence. The observations of Orth, which differ from those 

 of Langhaus, show that a difference of from sixteen and a 

 half to twenty-three feet not only causes no disturbance, but 

 under some circumstances may produce favorable reinforce- 

 ment of the direct by the reflected sound, and that with di- 

 minished intensity the difference may be somewhat greater, 

 but under no circumstances should it exceed thirty -three 

 feet. On the other hand, the intensity at a difference of 196 



