THE STRUCTURE OF THE NUCLEUS. 



53 



Id measuring the angular aperture « of the central white patch of the coronas, 

 the circular source of light was placed at the same distance (250 cm,) on one side 

 of the receiver as the eye on the other side. In this case the angles of incidence 



Fig. lb. 



Fig. 1. — Goniometers AND Appurtenances. la. Horizontal Form, i b. Vertical Form. 

 Fig. 2.— Condensation Chamker, N ; Filter, F ; Gauge, G ; Exhaust Pipe, E. 



and of aperture are equal, being I- the angle of diffraction, and the equation becomes, 



where \ is the wave length of the dark ring in question, n = 1 usually, a the 

 common angle of incidence and of aperture, and d the diameter of the diffracting 

 particle. If n = 1, equation (2) becomes in view of (1), 



,/ = "f (3) 



In the instruments used in this chapter R — lO.fj cm., and 20.1 cm., though larger 

 values are preferable. 



Instead of using equation (3), it is foi- many purposes advantageous to compare 



