C. Barus — Colors of cloudy condensation. 151 



be the intensity of the incident light and h (-04 to -05) the 

 reflection coefficient, then after a single transmission the inter- 

 ference maxima and minima are (1— #) 2 (l+& 2 )i" and (1 — k) 2 

 (1— ¥)I; they differ only very slightly. But if there be an in- 

 definite number of particles all of the same size available, 

 then this process is indefinitely repeated in such a way that 

 while the colored light is not extinguished, the admixed white 

 light becomes continually more colored. Hence after a suffi- 

 ciently great number of transmissions the emergent ray will 

 show intense color. Seen by reflected light the case is almost 

 the converse of this. For a single particle the masses which 

 interfere are (#_Tand h{l — TcfI) weaker but nearly equal, and 

 the interference is therefore very perfect. It is not, however, 

 capable of indefinite repetition for after each interference the 

 direction is reversed. The light which emerges in a direction 

 opposite to the incident ray must therefore have passed 

 through the particles, i. e. it has been brought to interference 

 both by reflection and by transmission, and its color is thus 

 virtually extinguished. 



The final point to be considered is the occurrence of black, 

 between brown and dark violet of the first order. Here, how- 

 ever, for relatively very small increase of the thickness of the 

 plate, the colors run rapidly from brown through red, carmine, 

 dark red-brown to violet. Hence these interferences are apt to 

 occur together and an opaque effect is to be anticipated. Par- 

 ticularly is this presumable, because the opaque field is coinci- 

 dent with the breakdown of the steady motion* of the jet. 



Thus it seems that the colors of cloudy condensation may 

 without serious error be interpreted as a case of Newton's 

 interferences by transmitted light. In so far as this is true 

 one may pass at once from the color of the field to the size of 

 the particles producing it ; and the dimensions so obtained 

 agree well with E,. v. Helmholtz's estimate made in accordance 

 with Kelvin's equation for the increase of vapor tension at a 

 convex surface. In the study of the condensation phenomena 

 vapor-liquid, the experimental power of a method, which is 

 adapted for instantaneous observation, and which for a certain 

 range of dimensions not only discriminate between vapor and 

 a collection of indefinitely small suspended water globules, 

 but actually defines their size, cannot be overestimated. An 

 account of my work together with other allied observations 

 will be given in the March number of the American Meteoro- 

 logical Journal. 



*I refer here to Osborne Reynold's work (Phil. Trans., Ill, p. 935, 1883) with 

 liquid jets, according to which after a certain critical velocity is surpassed, the 

 uniformly steady motion breaks up into eddying motion. 1 am also searching 

 for Reynold's lag phenomenon (1. c. p. 957). 



Am. Jour. Sci.— Third Series, Vol. XLV, No. 266. — February, 1893. 

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