LIGHT REACTIONS AND PHOTOTAXIS 597 



reactions to changes of intensity described above and shown 

 to be instantaneous and very easily reversible, are effected through 

 the medium of the eyes, this is not certain for the relatively slow 

 non-adaptive process of negativation. Should this process be 

 brought about by photic stimulation of the eye, we would have to 

 assume either a second slower, non-adaptive photochemical pro- 

 cess running alongside of the other, or else summatory action 

 of the quick and adaptive process, leading to a new and stronger 

 effect. These conclusions seem cogent, after the adaptive char- 

 acter of the reactions to changes of intensity of illumination 

 has been recognized. 



A fact which may at first sight speak against the photorecep- 

 tors as mediators of the process of negativation is the discovery 

 made b}^ Loeb, that ultra-violet light of a mercury lamp makes 

 the Balanus-larvae negative at a quicker rate, than sunlight, 

 especially if the latter is deprived of its ultra-violet rays by 

 means of a glass plate. The rays of shortest wave-length would 

 thus have a stronger negativating influence than all the other 

 rays put together. This maxirnum would not coincide with 

 that found for orientation. On the other hand, it is possible 

 that in the case of the ultra-violet rays we are dealing with a 

 special case influenced by the deleterious effect of these rays on 

 the entire protoplasm; that ultra-violet light acts directly on 

 the chemical processes of metaboUsm and stands apart in its 

 effect. It is shown by the following experiment, that ultra- 

 violet light has a strongly deleterious effect also on the Balanus- 

 larvae. 



Two blackened watch glasses containing animals were put in 

 diffused daylight or sunlight. One was covered with a strong 

 clean glass plate, the other was left uncovered. After a few 

 minutes in sunlight or one to tw© hours in strong diffused light 

 the nauplii in the uncovered dish were killed. Those in the 

 covered dish lived for hours. I have often repeated this experi- 

 ment. Every time a simple glass plate, which did not weaken 

 the visible rays ostensibly sufficed to retard the harmful effect 

 of strong light on the nauplii, showing thereby that the effect 



