318 Profs. Percy Frankland and Mai-shall Ward. 



an hour at least, the filaments developed from the spores in darkness 

 or behind the powerful shades (green and red glass) grew to from 

 125 to 187 times the length of the spore; those behind the bine and 

 ordinary glass only grew to 10 to 15 times the length of the spore. 



Moreover, it is interesting to observe that, so far as it goes (and I 

 do not wish to press it unduly, because a larger number of measure- 

 ments will be required to decide such a point) the red glass again 

 acted as an incubator during the early stages, which I attribute to its 

 acting as a trap for the dark heat-rays ; and the more pronounced 

 retarding action of , the blue glass so long as the stronger light could 

 penetrate it, fell somewhat after six or seven o'clock, when the dark- 

 ness set in. 



As before, I regarded the green as acting much like the red : it 

 cuts off much of the intensity of the light, and in so far is a shade, 

 while it transmits a light of such composition that the dark heat 

 rays are relatively in excess, and still some blue passes. Neverthe- 

 less, there is something puzzling about the behaviour of this green 

 glass which I cannot as yet explain. The same uncertainty of its 

 action prevails in other cases, as I have already stated. 



To test still further the action of various coloured screens, I 

 repeated the experiments with glass screens, taking a series of com- 

 parative thermometrical readings at the same time. The days 

 chosen (February 18 19) were very bright and clear, and great 

 care was taken to obtain the light used from the clear blue sky, to 

 the east of the sun, so as not to run any risk of direct insolation at 

 unexpected periods. 



Moreover, before beginning the experimental exposures, I made a 

 series of observations by placing the bulbs of thermometers between 

 the glass screens exactly in the place occupied by the moist culture- 

 chambers, and exposed them under all conditions. 



When the thermometer lying on the table under the south window 

 used for exposures, registered 25 C. in the sun, I found that the one 

 with its bulb over the mirror of the microscope registered 27 C.^ 

 when the light reflected from the blue sky at noon passed through 

 the blue glass : on then reflecting the sun's image on to the same bulb 

 between the blue glasses, the temperature, of course, rapidly rose, 

 and was at 32 C. ten minutes later. 



On repeating this with the bulb of a thermometer between the red 

 glasses, the temperature rose to 33 0. in the same time (10 minutes), 

 and rapidly fell to 28 C. when the mirror reflected only light from 

 the blue sky to the east of the sun. 



With the orange glasses, the temperature shot up to 33 C. in the 

 sun's image, and fell to 27 C. in 10 minutes on turning the mirror as 

 before ; and with the green glasses, the temperature rose to 30 C. in 

 the sun, and fell to 27 C. in the rays from the blue sky. 



