86 PRINGSHEIM^S RESEARCHES ON CHLOROPHYLL. 



daylight, and in ordinary sunlight, only a feehly intense light 

 to pass, which has much less effect than the same rays have 

 when acting directly in sunlight. By the yellow and green 

 screens employed (green glass or solutions of copper chloride 

 and potassium bichromate) the intensity of the light is dimi- 

 nished to a much less extent. The intensity of light action is, 

 in experiments with coloured screens, undoubtedly dependent 

 upon the illuminating power of the acting rays, and the 

 strikingly small effect given as the result of the operation of 

 blue light is to be explained in great part by the non-trans- 

 parency of the screen employed. Moreover, as the blue are 

 preferably and more strongly absorbed in the chlorophyll 

 than the green and yellow rays, they will act with less in- 

 tensity than these upon the cell contents protected by the 

 chlorophyll (that chlorophyll plays this part is one of the 

 results of the researches here described), and it is necessary 

 to bear this fact in mind when considering the effect of, and 

 in estimating the relative energy of rays of different colour. 

 In all experiments with blue rays the specific chlorophyll- 

 absorption must be compensated by using more intense 

 light. 



In studying photochemical effects under the microscope, 

 the simplest method is to concentrate sun^s rays upon the 

 object by means of a condensing lens under the stage, care 

 being taken that the object is kept accurately in the plane 

 of intense illumination. In the researches recorded here a 

 microscope with a double stage was used, the upper one for 

 the object vertically movable, the lower one fixed, and pro- 

 vided on the under side with a doublet-condenser of plano- 

 convex lenses, which gave an aiea of illumination 0"o5 mm. 

 in diameter. The light reflected from a heliostat was 

 directed by the mirror through this lens, by which it was 

 concentrated on the object. By means of clamps under the 

 fixed stage various absorptive screens — several kinds were 

 used — for shutting out the dark heat rays and for produc- 

 ing lights of different colours could be fixed between the 

 condensing lens and the mirror. These screens could also 

 be placed above the fixed stage. For the object a small 

 glass chamber was constructed, the floor of which was 

 always covered with a drop of water — a most necessary pre- 

 caution. Special metallic chambers, with glass floor and 

 movable lid, were used for studying the effects of different 

 gases, A dark screen for completely shutting off the light 

 from the heliostat, and a neutral-tinted ocular, complete the 

 special apparatus employed. 



A considerable amount of heat is produced by the intense 



