600 WOLFGANG F. EWALD 



sufficient exactitude tfie effect of the extinction of the mono- 

 chromatic light from the effect of its wave length. In fact the 

 effect varied, especially in the green, according to the opacity 

 of the solution. 



We may state in conclusion, that two maxima of negativating 

 effect were found inside the visible part of the spectrum: one in the 

 green and one in the violet, the minimum being in the red. If we 

 were to show the negativating effect of hght rays by a curve, we 

 should have to begin with a maximum in the ultra-violet slowly 

 faUing towards the blue, rising again to the yellow-green and 

 falling steadily towards the minimum in the red. Ths nature of 

 the curve makes it probable, that we have to deal with at least 

 two interacting effects, one of which may operate by the med- 

 ium of the eyes (maximum in the green) while the other acts on 

 some body substances through the cuticle directly (maximum in 

 the ultra-violet). 



I cannot, however, refrain from mentioning the fact, that this 

 result was not always attained. Especially during experiments 

 in which the pure, strong and broad spectrum of the Rowland 

 grating was replaced by the considerably smaller, weaker and 

 less pure spectrum of a carbon bisulphide prism, the effect in 

 the green was invariably so small, that I stated a minimum 

 instead of a maximum in this part. The negativating influence 

 of the yellow and even the red rays was sometimes stronger 

 than that of the green ones, whereas the effect on the motor 

 reflexes was also in this case strongest in the green. 



To conclude, I will mention a few experiments on the dele- 

 terious effect of concentrated monochromatic light. If the ani- 

 mals contained in the color-filter glasses described above were 

 exposed for about two hours to full sunhght, they were killed 

 first in the violet, later in the green and the blue and not at 

 all in the red. To the eye the red solution seemed most trans- 

 parent, the violet nearly opaque. The permeability to ultra- 

 violet rays was probably equally small in all glasses. 



I shall now proceed to describe a series of experiments carried 

 out in order to determine the effect of variations of the tem- 

 perature and the chemical composition of the water on the light 



