126 RESEARCHES ON FUNGI 



coinciding with the septum's centre, in consequence of which the 

 other spot hes excentrically over the septum ; or (2) the two spots 

 as a whole take up a symmetrical position over the septum, so that 

 their common centre (situated in their overlapping parts, cf. Fig. 

 59, D) concides with the septum's centre. If one spot only comes 

 to lie symmetrically on the septum, the fruit-body must point to 

 but one of the two sources of light ; whereas, if the two spots act 

 as one and, as a whole, lie symmetrically over the septum, the 

 fruit-body must point in a direction which bisects the directions of 

 the two sources of light. 



A fruit-body which, in accordance with the possibility (2) as 

 just described, is supposed to be directed between the two equal 

 sources of light has been represented in Fig. 59. At B, the two 

 beams of light which make an angle of 5° with one another are 

 indicated by the lines a and b and the axis of the fruit-body, which 

 bisects the angle, is indicated by the line x. At C is a construction 

 diagram of the fruit-body having its axis x parallel to a; in B and 

 being illuminated by one of the two beams of light, namely, the 

 one shown by a in B. The rays of this beam, which make an angle 

 of 2*5° with the axis of the subsporangial swelling, are shown in 

 C in the air at a and b and, after refraction, in the swelling where 

 they converge and form a spot of light on the septum excentrically 

 on its right-hand side. To avoid confusion, the other beam of 

 light with rays parallel to 6 in B has not been represented in C, but 

 it can be readily imagined as it would be just like the beam actually 

 represented, but reversed in position, The spot of light formed by 

 this unrepresented right-hand beam, like the other spot, falls on 

 the septum excentrically, but with its centre a little to the left of 

 the septum's centre instead of a little to the right. The con- 

 struction diagram C indicates quite clearly that the two spots 

 of hght must overlap to some extent in the manner shown at 

 D and that, as a whole, they can lie symmetrically on the 

 septum with their common centre resting upon the centre of the 

 septum. 



If the angle between the two beams of light were to be decreased 

 gradually from 5° to 1° the overlapping of the two spots of hght in 

 the subsporangial swelling would increase from that indicated in 



