THE PILOBOLUS GUN AND ITS PROJECTILE 







95 



Fig. 48.- — Diagram showing how a beam of parallel light-rays is refracted within 

 the subsporangial swelling of Piloholus Kleinii. The disc represents a much 

 enlarged transverse section of a swelling which had its axis horizon- 

 tally directed. The rays of light 1-7 are passing upwards from the plane 

 mirror of a microscope. The rays on entering the swelling, assuming the 

 refractive index from the air to the cell-sap is 1 • 34, must be refracted as shown, 

 so that they all emerge between c and d, the parts of the swelling s and s receiving 

 no light whatever. An observer looking down the microscope in the direction 

 of the arrow o should, therefore, theoretically, perceive the upper wall of the 

 swelling brightly lighted between c and d and black between a and c and between 

 b and d. The lighted and unlighted portions of the wall are shown projected 

 above, the dark regions being represented by e-g and h-f and the illuminated 

 one by g-h. When actual observations were made with a beam of parallel 

 light rays, it was found that the dark regions of the wall were as projected at 

 i-k arid l-j and the illuminated one as projected at k-l. The theoretical results 

 showTi in the line e-J agree very well with the practical results shown in the 

 line i-j. This agreement justifies one of the assumptions underlying tlie 

 construction of Figs. 46, 47, and 59, namely, that the refractive index for light 

 in passing from air to cell-sap is approximately 1 • 34. 



