12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



FOURTH EXPERIMENT 



The apparatus was completely overhauled for the fourth experi- 

 ment. This time a more trustworthy thermostat was used with a 

 safety device for cutting of( the heater and lights in case of accident. 

 An electric clock was connected with the lighting system in such a 

 manner as to indicate any length of time the apparatus was not func- 

 tioning properly when the oliserver was ahsent. 



Johnston's (1932) work with tomato plants grown under Mazda 

 lamps indicates that a large proportion of infrared radiation present 

 in this type of illumination is injurious to the plants. Heat-absorhing 

 filters give more normal growth and physiological response, and where 

 the excessive infrared radiation from the lamps is absorbed by a 

 solution of copper sulphate, the plants are more normal. For this 

 reason a solution of copper sulphate (1.007 specific gravity at 80° F.) 

 was used in place of distilled water in the circulating system that 

 controls the temperature of the four baths. 



In the third light-intensity experiment on page 9 it was found that 

 Stichococcus bacillaris increased in proportion to the increase in light 

 intensity when the intensities measured through water were 3.76, 

 1 1.5, and 34.1 microwatts/mnr. In this experiment, as well as in the 

 fifth one, the intensity measured through the copper sulphate solution 

 was 25 microwatts/mm-'. Since the intensity as measured through 

 the copper sulphate solution is less than the next to the highest light 

 intensity used in the third light-intensity experiment, we know that 

 the light intensity in the filter experiments is favoral)le to the growth 

 of these algal cells. 



The copper sulphate solution entered each separate Ijath through 

 a flaring glass tube, oyer the end of which was placed a bag of buck 

 toweling. These bags strained out most of the bubbles caused by the 

 central pump, as well as any dirt or grease present, thus keeping the 

 copper sulphate solution clear in each bath. 



Mazda lamps were used under each bath and so arranged at 

 distances from the glass bottom of the bath that the light intensity 

 was the same under each of the duplicate sets of 12 filters as mea- 

 sured by the thermocouple. Frequent readings of the light intensity 

 were made during the experiment to insure the similarity of the in- 

 tensity under the filters, and when necessary the distances of the 

 lamps from the bottom of the bath were changed or the lamps re- 

 placed by new ones. The voltage was read at the same time the in- 

 tensities were measured, since the voltage fluctuated slightly from 

 day to day thus causing a difference in intensity. The Erlenmeyer 



