NO. 23 STIMULATION OF ALGAE BY ULTRAVIOLET MEIER 7 



was made in similar fashion to the initial count. A separate pipette 

 was assigned to each flask. In this manner equal drops were obtained 

 from each culture. The quartz irradiation tube contained generally 

 24 drops of inoculum, which were divided between the two Erlenmeyer 

 flasks in the manner described above. The number of cells per drop 

 of inoculum for each culture of the same experiment was fairly uni- 

 form. The number of cells per drop of inoculum varied in the differ- 

 ent experiments. 



To insure counting every cell in a drop, a special microscope slide 

 was etched for the purpose by L. A. Fillmen, of the Division of Ra- 

 diation and Organisms, in the following manner : The slide was coated 

 with a thin layer of beeswax and then ruled into rectangles on the mill- 

 ing machine with a sharp-pointed tool. The lines were 1 mm. apart 

 lengthwise and 4 mm. apart crosswise. The lines were etched into 

 the glass by placing a drop of hydrofluoric acid with a glass rod on 

 the slide, and by spreading the drop with the glass rod into the grooves 

 where it rested for a fraction of a minute. The acid was washed off 

 with water, the beeswax was scraped off with a sharp flat tool, and 

 the slide was cleaned. 



The special pipettes made by L. B. Clark were drawn to a point so 

 that a drop from each could be covered completely by a No. 2 A, f -inch 

 cover glass. By using a euscope attachment to the microscope and a 

 mechanical stage, it was a simple matter to count every cell on the slide 

 with either the high-power or the low-power objective and a No. 5 

 ocular. 



THE GROWTH CHAMBER 



During the early experiments conducted in 1937 the algae in the 

 Erlenmeyer flasks were grown during the 2 weeks following irradia- 

 tion in natural conditions of day and night at variable room tempera- 

 ture in the north window on the eleventh floor of the Smithsonian flag 

 tower. These environmental conditions of growth proved to be en- 

 tirely too variable, as shown by the results for the experiments with 

 algae grown under uncontrolled conditions presented in tables 1-4. 



To arrange an ideal environment for the growth of the cultures after 

 irradiation, an electric refrigerator was equipped with a thermostat, 

 which regulated the temperature of the inner chamber at a constant 

 temperature of 24 C. during the day and 22 C. at night. The new 

 fluorescent daylight lamps were tested and proved to produce better 

 growth conditions for the algae than varying daylight (see pi. 1). 

 A set of four of these daylight lamps, each of which was 15 watts and 



