IJ 51 



it 18 itself entirely destitute of any sensitivity for pholo-electricity. 

 Tiiis renders it possible to permeate it with all sorts of colloids 

 occurring in the animal tissue and to observe the effect of light t)n 

 these colloids in divers ditfusion and under ditFerent conditions. Only 

 the slight permeability of the silicic acid for ultraviolet light is in 

 some respects an impediment. For a deeper penetration of the ultra- 

 violet light silicic acid must be replaced by a gelatin gel, which is 

 itself, as already observed, little sensitive to light. Conversely, 

 covering the gel with a superficial culture will act like a screen. 

 This was evident when e.g. we covered a gelatin gel with a closely 

 plated cultuie of luminous bacteria. The result was a distinct 

 deci'case in the sensitivity to photo-electricity of the gelatin gel. 



Our conclusion is that the photo-electricity of animal tissues 

 depends on the sols, gels and solid substances (fibers producing 

 donble refraction, crystals, liquid crystals) contained in it. The 

 experiment with the gels affords an opportunity of combining these 

 component parts in any given cpiantity. If only an effect on the 

 superficial layers is aimed at, silicic acid must be resorted to, as 

 it has of itself no sensitiveness for photo-electricity ; while gelatin 

 must be used if the inquirer has in view the effect upon the dee|)er 

 layers. 



