94 RESEARCHES ON FUNGI 



protoplasm taken together into cell-sap/ 0-893. Where the proto-- 

 plasm formed a thin even layer on the cell-wall, the cell-wall and 

 protoplasm were treated as forming a very thin negligible plate, 

 and the rays were regarded as being refracted directly from the air 

 into the cell-sap (refractive index 1-34). On the other hand, where 

 the protoplasm was heaped up around the top of the subsporangial 

 swelhng (rays 1,2, and 3 in Figs. 46 and 47), the path of each ray 

 was calculated first from the air into the cell-wall and protoplasm 

 taken together (refractive index 1 • 5) and then from the cell- wall 

 and protoplasm taken together into the cell-sap (refractive index 

 0-893). 



An experiment which proves that the refractive index from air 

 into the cell- sap of Pilobolus is, as assumed above, approximately 

 equal to the refractive index from air into water, namely, 1 • 34, was 

 made by comparing the actual width with the calculated width of 

 the central illuminated part of a median cross-section of a sub- 

 sporangial swelhng when the swelling is placed with its axis hori- 

 zontal in air under the microscope, is illuminated from below with 

 a beam of parallel light rays, and is viewed from above. In Fig. 48, 

 the cross-section of the swelhng is represented by a circle, and the 

 observer is supposed to be looking downwards on it in the direction 

 shown by the arrow o and to be observing the width of its illuminated 

 central part. When an actual beam of parallel light rays was 

 reflected upwards from the mirror, the illuminated central part of 

 the median cross-section of the swelhng had a width of k-l and the 

 lateral dark parts widths of i-k and l-j. When a theoretical beam 

 of parallel Ught rays was represented as being refracted into the 

 interior of the swelling and when, in calculating the paths of the 

 rays through the cross-section, 1 • 34 was used as the refractive 

 index from the air into the cell-sap, the illuminated central part of 



1 Knowing the refractive index from air into cell-wall and protoplasm and 

 from air into cell-sap, the refractive index from cell-wall and protoplasm into cell- 

 sap can be readily calculated from the equation : 



a X fc X c = 1 

 where a ^ the refractive index from air into cell-wall and protoplasm (1-5), 



b = the refractive index from cell-wall and protoplasm into cell-sap, and 

 C — the refractive index from cell-sap into air (1/1-34). 



