THE PILOBOLUS GUN AND ITS PROJECTILE 103 



Since the refractive index for light passing from air into the 

 cell- wall of Pilobolus, namely, 1-5, is approximately equal to the 

 refractive index from air into a transparent medium given in 

 Table I, namely, 1 • 55, we may apply the data embodied in Table I 

 in our study of Pilobolus. 



The angle of incidence for each of the six rays, Nos. 1-6, repre- 

 sented as striking the subsporangial sweUing in Fig. 46 (p. 91) 

 was carefuUy measured, and then the percentage of light reflected 

 from each ray at the surface of the swelling was estimated approxi- 

 mately from the data given in Table I, with the result shown in 

 Table II. 



Table II 



Light reflected at the Surface of the Subsporangial Swelling of Pilobolus 



The greater the percentage of the light reflected, the feebler 

 will be any ray after refraction. From the data embodied in 

 Table II, therefore, we may draw the conclusion that the rays 

 refracted through the subsporangial swelling shown in Fig. 46 

 diminish in brightness in order from No. 1 to No. 6, the brightest 

 ray being No. 1, and the dullest ray being No. 6 (theoretically this 

 ray is not refracted at all since its angle of incidence is not less 

 than 90°). Since, as shown in Fig. 46, the refracted rays diverge 

 before impinging upon the basal protoplasm, it is evident that the 

 brightest part of the spot of light produced at the base of the sub- 

 sporangial swelhng is centrally situated and rests on the inner edge 

 of the protoplasmic septum ; and that the brightness of the spot 

 of light diminishes centrifugally. The protoplasmic septum pro- 

 trudes so far into the vacuole that it is just able to catch most, or 



