CHAPTER II. 



GENERAL UHSElIVATlUNS KKLATING TO COItONAS AMI AXIAL COLOKS. 



INTRODUCTORY. 



1. Purposes of the present cluipter. — I have stated above that it is the chief 

 purpose of the present volume to throw light on the structure of the nucleus from 

 measurements made of the apertures of the coronas of cloudy condensation, vari- 

 ously produced. As these coronas are not merely expansions of the well-known 

 form, but })resent complications which make the direct measurement of aperture 

 beyond a certain order of magnitude of but little value for classification, it was 

 thought necessary to enter somewhat at length into the investigation of these phe- 

 nomena. Incidentally, an outstanding inquiry of my first volume, as to the rela- 

 tion of steam jet oi- (axial) color tube colors and coronal colors is thereby definitely 

 auswei'ed. 



2. Classification of the experiinents. — The object of the present chapter is, in 

 the first place, to map out the sequence of coronas in case of water vapoi', in terms 

 of the numbers of particles producing them, relatively. The extreme diversity of 

 coronal display seen in moist nucleated air lends itself well to a geometric method 

 of classification when the colors are produced by successive exhaustions. The 

 classification is thus primai'ily suggested by experiment. 



In the second place, I shall make certain theoretic deductions from the time 

 losses of nuclei observed, which clear up some moot points left in abeyance in my 

 eai'lier experiments on the same subject. 



I purpose in the third place to contrast the color of the central patch of the 

 coronas with the axial colors seen in the steam jet, or under like circumstances 

 with even greater saturation, in the adiabatically exhausted drum. The coronas 

 must in large measure be diffraction phenomena; the axial colors cannot be so 

 explained, but are evoked by some unknown kind of harmonic absorption. The 

 contrast is sharply brought out by the experiments. 



Finally, I shall make an estimate of the absolute dimensions of the water 

 particles in action and of their number, and indicate a method which will be 

 pursued in the next chapter (III), in a quest for definite absolute results. 



APPARATUS. 



3. Tubes.— In my first experiments, tubular apparatus were used, after the 

 manner of Kiessling and Aitken. But in these instances the colors are too fleet- 

 ing and, if obtained artificially with nuclei, are apt to be too dull for good dis- 

 ss 



