316 Intelligence and Miscellaneous Articles. 



second order, seen by transmitted white light under normal inci- 

 dence. Thus it is worth inquiring whether small globules of water, 

 when white light is normally transmitted, affect it like thin plates. 

 For a given homogeneous colour, if I be the intensity of the incident 

 light and Tc ( 04 to -05) the reflexion-coefficient, then after a single 

 transmission the interference maxima and minima are 



(l-l>)Xl + k 2 )I and (1-F) 2 (1-& 2 )I; 



they differ only very slightly. But if there be an indefinite number 

 of particles all of the same size available, then this process is in- 

 definitely repeated in such a way that while the coloured light is 

 not extinguished, the admixed white light becomes continually 

 more coloured. Hence, after a sufficiently great number of trans- 

 missions, the emergent ray will show intense colour. Seen by 

 reflected light, the case is almost the converse of this. For a single 

 particle the masses which interfere are (M and k{l — k) 2 l) 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 wilich 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 reflexion and by transmission, and its colour 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, however, for 

 relatively very small increase of the thickness of the plate, the 

 colours run rapidly from brown through red, carmine, dark red- 

 brown to violet. Hence these interferences are apt to occur to- 

 gether and an opaque effect is to be anticipated. Particularly is 

 this presumable, because the opaque field is coincident with the 

 breakdown of the steady motion * of the jet. 



Thus it seems that the colours of cloudy condensation may, 

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

 ferences by transmitted light. In so far as this is true, one may 

 pass at once from the colour of the field to the size of the particles 

 producing it ; and the dimensions so obtained agree well with B,. 

 v. Helniholtz's estimate made in accordance with Kelvin's equation 

 for the increase of vapour-tension at a convex surface. In the 

 study of the condensation phenomena vapour-liquid, the experi- 

 mental power of a method, which is adapted for instantaneous 

 observation, and which, for a certain range of dimensions, not only 

 discriminates between vapour 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 Meteorological Journal.' — Silliman's Journal, February 

 1893. 



* I refer here to Osborne Reynolds' work (Phil. Trans, iii. 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. 

 I am also searching for Reynolds' lag phenomenon (I. c. p. 957). 



