542 Prof. Hood on measuring the average size of very fine Particles. 



When the difficulty of obtaining an approximate measurement 

 of the size of the particles of lampblack is considered, it is sur- 

 prising to see how nearly the calculated difference in path ap- 

 proximates to half a wave-length of the light in question. 



I found that a surface of magnesia, produced by smoking a 

 glass plate to opacity with burning magnesium wire, also re- 

 flected light in the same way at very oblique incidences. It was 

 ascertained that the final tint was red, and that the red rays 

 themselves disappeared at 11°. The size of the particles of mag- 

 nesia Was estimated at '000036. Long after the measurements 

 had been obtained I calculated the difference of path for the 

 interfering rays; this was found. to be 



•000014, 

 wave-length of C -000026, 



giving a still nearer approximation to a difference of half a wave- 

 length. 



These experiments, then, seem to point out the correctness of 

 FresneFs theory ; and we should, I think, be justified in revers- 

 ing the process, and using the angle of disappearance of the red 

 ray in connexion with the known wave-length of this ray, for 

 the purpose of calculating the average size of small particles or 

 the average depth of fine scratches or furrows. 



I give below the calculated values of the average size of the 

 particles of lampblack and magnesia :■*- 



Lampblack from paranine . . -0000188 calculated. 

 „ „ . . -0000146 measured. 



Size of particles of magnesia . -0000338 calculated. 

 „ „ . -0000361 measured. 



Some experiments were made on the angle of disappearance 

 of the red ray with lampblack produced by the burning of dif- 

 ferent substances : where the figures are connected by a bracket 

 it is intended to indicate that the two angles were obtained from 

 the same portion of the plate. 



x , -. , T , , , Lampblack from a 



Lampblack Lampblack golut £ n of h , its of 



from stearme. from campnor. turp8ntine in alcohol. 

 o o o 



18 " 



•251 16 \ 22\ 



•75 J 15-9J 21 J 



1 15-11 



J 15-4 J 



18- 



16 

 16 



17-25 15-6 21 



It would appear from these last experiments that the average 

 size of the particles of lampblack from burning camphor is 

 somewhat greater than from paranine, while in the case of 

 "burning fluid ** the particles are smaller. 



