Deckmbke 7, 1900. ] 



SCIENCE. 



887 



two species of Orchestidae found at Woods 

 Holl, Mass., Talorchestia longicornis and Orchea- 

 tia agilia. Talorchestia was found to be posi- 

 tively phototactic at all times both in strong 

 and in weak light. Orchestia agilia is somewhat 

 less strongly positive since, for a short time 

 after it is taken from the dark, it becomes nega- 

 tively phototactic, but exposure to the light 

 soon makes all the individuals strongly positive 

 and they remain so even in direct sunlight. 

 Both these species when exposed to direct sun- 

 light remain positively phototactic until over- 

 come by the heat of the sun. Yet the animals 

 when they come to rest are found in shaded 

 spots and, during the daytime, remain unex- 

 posed to the light ; they are photophobic, but 

 positively phototactic. Observations were made 

 on twenty-two species of aquatic amphipods, all 

 of which were found to be negatively photo- 

 tactic. It was found that when the terrestrial 

 Orchestia agilia were thrown into sea water their 

 phototaxis immediately changed from positive 

 to negative ; when taken out of the water the 

 reverse change occurs, and this change was 

 shown to be independent of temperature. 



Orchestia agilis, when brought from strong 

 light, in which it is strongly positive, to weak 

 light, immediately becomes strongly negative, a 

 result which, it is believed, has been observed 

 in no other form. When left in weak light the 

 negative phototaxis disappears and all the 

 specimens become positive again. This result 

 was shown to be independent of temperature. 

 Specimens rendered negative by being brought 

 into a dimly lighted room after exposure to 

 strong light become positive more quickly if 

 the temperature is raised. Exposure to dark- 

 ness or to very strong light renders Orchestia 

 agilis temporarily negative in light of medium 

 intensity. 



In Talorchestia, Orchestia, and several species 

 of insects that are positively phototactic it was 

 found that, when one eye is blackened over, the 

 animal travels in circles with the unblackened 

 eye looking towards the center. In several 

 negatively phototactic forms blackening one eye 

 was found to produce circus movements in the 

 other direction, the unblackened eye looking 

 away from the center. Orchestia agilis may be 

 caused to execute circus movements in the one 



or the other direction according to whether it is 

 placed in air or water. H. 8. Jennings, 



Secretary. 



DISCUSSION AND C0REE3P0NDENCE. 

 THE BLECTEICAL THEORY OF GRAVITATION. 



Professor Fessenden in a recent number 

 of Science discusses the nature and velocity 

 of gravitation. There is, no doubt, something 

 of value in Professor Fessenden's suggestions 

 and much that is new. However, the explana- 

 tion of gravitation which Professor Fessenden 

 offers is by no means so adequate as would ap- 

 pear from Professor Fessenden's discussion. 



In the first place, Quantitative Mathematics, 

 as Professor -Fessenden styles his papers on 

 Dimensions, has little or nothing to do with the 

 thing. The present writer makes this state- 

 ment not entirely as an expression of his own 

 opinion, but quite as much as an expression of 

 opinion of every physicist with whom he has 

 talked on the matter. Professor Fessenden 

 claims to have derived numerical functional re- 

 lations, with the aid of his ' Qualitative Mathe- 

 matics,' and this is believed on definite rational 

 grounds by most physicists to be impossible. 



One of the best examples of the application 

 of ' Qualitative Mathematics ' is its application 

 by Lord Rayleigh {Philosophical Magazine, Octo- 

 ber, 1899), in his ' Investigation on Capillarity.' 

 In this instance the weight of a drop of water 

 falling from the end of a glass tube is shown to 

 be a certain function of radius of tube, density 

 of water, acceleration of gravity and surface 

 tension of water multiplied by an unknown func- 

 tion of these quantities having zero dimensions in 

 length, mass and time. This unknown and un- 

 knowable function — to be determined by experi- 

 ment only — Fessenden seems to lose sight of 

 in his 'Qualitative Mathematics.' The reader 

 who wishes to get the gist of Professor Fessen- 

 den's suggestions as to the nature of Gravity 

 may therefore, as it seems to us, ignore what 

 he says of, and is led to say by ' Qualitative 

 Mathematics.' 



The following is a brief and fairly simple out- 

 line of the electrical hypotheses which are now 

 held by physicists in the attempt to explain the 

 ultimate constitution of matter and the nature 

 of inertia and of gravitation. 



