336 C. Barus — Apertures of Coronas. 



tion for coronas of the higher orders, where d and D are pos- 

 sibly both implicated in producing coronal effects. If one 

 builds up a system of glass plates each sprinkled with lyco- 

 podium particles, the diffraction pattern which is finely annu- 

 lar for a single plate, is a mere blur for ten plates placed within 

 a linear foot for instance, without changing the aperture appre- 

 ciably. If the source of light and the eye are both distant, 

 the coronas gradually lose sharpness and soon cease to be 

 measurable as the number of plates increases. This indicates 

 the nicety of distribution and the equality of diameter that 

 must be met with in case of cloud particles, but it leaves the 

 question open whether the distance apart of particles is not 

 from the outset a consideration. 



What is further menacing is the distortion produced by 

 spherical and cylindrical vessels, the surfaces of which are 

 rarely quite concentric. In my work with globes, I assumed 

 that if the annuli showed no elliptic distortion and were small 

 in aperture as compared with the aperture of the globe, distor- 

 tion could be neglected. It is questionable, however, if this 

 observation is vouched for, since the apertures of coronas are 

 peculiarly sensitive to refraction, particularly when the dis- 

 tances of eye and source from the receiver are purposely 

 chosen large. § 3. 



Again, the quantity m is dependent on temperature to a 

 marked degree. It is necessary, therefore, to refer coronas to 

 a standard temperature as well as to a given degree of super- 

 saturation, and the correction is particularly important if the 

 coronas are to be used in estimating the number of particles. 



Finally, the ratio of densities before and after exhaustion is 

 a seriously difficult datum to determine, for it depends on the 

 degree to which adiabatic conditions have been attained. It is 

 here that the work is liable to be discrepant. Hence a deter- 

 mination of apertures has an ulterior value, for it is not 

 improbable that the two series of results will mutually inter- 

 pret each other. The present paper bears out this surmise. §7. 



2. Apparatus and I prelimin ary results. — The tables investi- 

 gated contain a preliminary survey* of the sequence of coronas, 

 their apertures (s) and the number of particles in per cub. cm.) 

 of specified diameter (d) encountered. The data for diameter 

 and number, d and n, are taken from my work on successive 

 exhaustion, where the experiments are largely non-optical, and 

 they are compared with the corresponding data, d' and n\ which 

 follow from measurements of aperture. The eye and source of 

 light are distant 1 and 3 meters, respectively, from the conden- 



* The tables are withdrawn for brevity, since the data used in this paper 

 are sufficiently given in the chart. 



