1920] HURD—FUCUS SPORES 29 



In the experiments to be described, Wratten light filters were 

 used, each of which was fitted as a window in the end of B dark 

 box. Each transmitted only a narrow range of wave lengths, but 

 together they embraced the whole of the visible spectrum. The 

 wave lengths to which each screen was transparent were deter- 

 mined by testing the light transmitted by each with a direct vision 

 spectroscope with a wave-length scale attached. Thus the quality 

 of the light stimulus acting in each box is accurately known. The 

 dark boxes were 10X13 cm. and 8 cm. high, in one end of each of 

 which a hole was cut so that one of the light filters, 5X5 cm., might 

 be fitted into it. The boxes were made light-tight with tightly 

 fitting covers, and were painted black inside to guard against 

 reflections within the box. The dishes used for the cultures were 

 either the ordinary Petri dishes or special dishes made of micro- 

 scope slides cemented together with zinc cement so as to make 

 shallow oblong dishes 7.5x2.5cm. and 1 cm. deep. It was at 

 first deemed necessary to use such flat-sided dishes in order to pre- 

 vent possible complications from reflected and refracted light in 

 the curving sides of round dishes, but later it was found that the 

 same results were secured in the Petri dishes. In order to expose 

 more than one dish behind each screen so that none would be 

 shaded by another, a rack was made to fit inside the box with cleats 

 projecting inward from the ends so that three dishes could be 

 slipped into it, one above the other. The light, entering through 

 the screen at the end of the box, fell equally on the one expand 

 side of each of the three dishes. The rack containing the dishes 

 could easily be lifted out and carried to the microscope for exami- 

 nation without disturbing the material under investigation. 



The source of light first used with the filters was th electric 

 arc. The advantage of this light over any other artificial light is 

 that it gives all the wave lengths of the visible spectrum, so that 

 all the filters could be used in each exposure, insuring identical 

 conditions of temperature, constancy of illumination, etc. The 

 disadvantages are several. In the first place, the intensity is 

 constantly changing as the carbons burn and the arcs get longer, 

 and the lessening of the intensity may not be the same for all the 

 wave lengths. In the second place, fluctuations in the current 



