214 Messrs. Lodge and Clark on Dusty Air in the 



wave-lengths there given were not conjectural, but directly 

 determined by the only practical method from the use of a 

 grating. They were the result, in fact, of the measurements 

 I have just described, and were specially intended to give 

 information about the unknown region extending beyond the 

 limit of M. Becquerel's researches, such as the great newly 

 discovered band O, for instance, which stretches from wave- 

 lengths 1^80 to P-90, while M. Becquerel's furthest band, as 

 I have said, is at 1^*48 (according to him; but according to 

 my measures more nearly at l /x, 38). The present memoir 

 will show what degree of reliance may be placed on these 

 measurements. 



It is understood that a photographic map of the spectrum to 

 l^'G, and therefore covering the ground of M. Becquerel's 

 paper, but not extending as far as my XI, will shortly be pub- 

 lished from the joint labours of Professor Rowland and Captain 

 Abney; and as their results will probably be more exact than 

 the preliminary explorations in which either M. Becquerel or 

 myself have been engaged, we may await its appearance for 

 the determination of a part, at least, of the points in question. 



I would call attention to the fact that M. Becquerel has 

 stated that the furthest band known to him in Sept. 1883 

 (except from my own researches) had a wave-length of not 

 over 1^50, according to his own estimate. 



XXVI. On the Phenomena exhibitedby Dusty Air in the neigh- 

 bourhood of strongly Illuminated Bodies. By Oliver J. 

 Lodge and J. W. Clark *. 



Part I. 



[Plate VII.] 



IN the course of a lecture on Dust and Disease given at the 

 Royal Institution in 1870, Dr. Tyndall calls attention to, 

 and fully illustrates by experiment, a dark or dust-free region 

 which he had observed in the convection-currents rising from 

 hot solids placed in the path of a powerful beam of light (see 

 Proc. Roy. Inst. vol. vi. p. 1, also ' The Floating Matter of the 

 Air '). He describes this dark stream as of surprising sharp- 

 ness and definiteness, especially when it is seen above an 

 ignited platinum wire, the line of sight being parallel to the 

 wire but at right angles to the beam. Dr. Tyndall also gave 



* Communicated by the Physical Society, an abstract having been read 

 at the Meeting on the 9th of February. 



