322 



natVre 



\_yan. 31, 1889 



secondary induced sparks were watched for. By using a 

 second mirror like the first to catch the parallel rays and 

 reconverge them to a focus, the effect could be appreciated 

 at a distance of 20 yards. If the receiving mirror were 

 rotated through a right angle, it lost its converging power 

 on this particular light. 



Apertures in a series of interposed screens proved that 

 the radiations travelled in straight lines (roughly speaking, 

 of course). 



A gridiron of metallic wires is transparent to the 

 waves when arranged with the length perpendicular to 

 the electric oscillations, but it reflects them when rotated 

 through a right angle, so that the oscillations take place 

 along the conducting wires ; thus representing a kind of 

 analyzer proving the existence of polarized light. The 

 receiver itself acted as analyzer, however, for if rotated 

 much it failed to feel the disturbance. 



Conducting sheets, even thin ones, were very opaque to 

 the electrical radiation ; but non-conducting obstacles, 

 even such as dry wood, interrupt it very little, and Dr. 

 Hertz remarks, " not without wonder," that the door 

 separating the room containing the source of radiation 

 from that containing the detecting receiver might be 

 shut without intercepting the communication. The 

 secondary sparks were still observed. 



But the most crucial test yet applied is that of refrac- 

 tion. A great prism of pitch was made, its faces more 

 than a yard square, and its refracting angle about 30°. 

 This being interposed in the path of the electric rays, 

 they were lost to the receiver until it was shifted con- 

 siderably. Adjusting it till its sparks were again at a 

 maximum, it was found that the rays had been bent 

 by the pitch prism, when set symmetrically, some 22° out 

 of their original course, and hence that the pitch had an 

 index of refraction for these 2-foot waves about 1 7. 



These are great experiments. As I write, they are but 

 a month old, and they are manifestly only a beginning. 

 They are very simple: I have repeated some of them 

 already. They seem likely to settle many doubtful points. 

 There has been a long-standing controversy in optics, 

 nearly as old as the century, as to whether the direction 

 of the vibrations was in, or was perpendicular to, the plane 

 of polarization ; in other words, whether it was the 

 elasticity or the density of the ether which varied in 

 dense media ; or, in Maxwell's theory, whether it was 

 the electro-magnetic or the electrostatic disturbance that 

 coincided with that plane. This point has indeed by the 

 exertion of extraordinary power been almost settled 

 already, through the consideration of common optical ex- 

 periments ; but now that we are able electrically to pro- 

 duce radiation with a full knowledge of what we are 

 doing, of its directions of vibration and all about it, the 

 complete solution of this and of many another recondite 

 optical problem may be expected during the next decade 

 to drop simply and easily into our hands. 



We have now a real undulatory theory of light, no 

 longer based on analogy with sound, and its inception and 

 early development are among the most tremendous of the 

 many achievements of the latter half of the nineteenth 

 century. 



In 1865, Maxwell stated his theory of light. Before the 

 close of i«88 it is utterly and completely verified. Its full 

 development is only a question of time, and labour, and 

 skill. The whole domain of Optics is now annexed to 

 Electricity, which has thus become an imperial science. 



Oliver J. Lodge. 



A JAMAICA DRIFT-FRUIT. 



TN 1884, I made a collection of drift-seeds and seed- 

 ■^ vessels washed ashore on the spit of land inclosing 

 Kingston Harbour, Jamaica, called the Palisadoes. The 

 collection was sent to Kew, and it was utilized by Mr. 



Hemsley in the appendix to the " Botany of the Challenger 

 Expedition," vol. i. pp. 277-304, when discussing "the 

 oceanic dispersal of plants. In the collection there was 

 a fruit I had not seen before, and which was equally new 

 to Mr. Hemsley and the other officers at Kew. There 

 were several specimens of this fruit obtained, and these 

 were afterwards placed in the Kew Museums amongst 

 other drift-fruit, to wait until sufficient material was- 

 forthcoming to lead to their identification. 



The general character of the drift-fruits collected at 

 Jamaica points to the conclusion that they were brought 

 by the Gulf Stream current from the mouths of the- 

 Amazon and Orinoco. Jamaica lies directly in this 

 current, and for a long period fruits unknown in the 

 island itself have been collected on its shores. The most 

 striking of these is the fruit of Manicaria saccifera, a 

 palm confined to Trinidad and the mainland of South 

 America. Sloane (" Natural History of Jamaica," vol. ii. 

 p. 186) noticed this fruit more than two hundred years 



Representation of a Jamaica drift-fruit (natural size). 

 2, cross-section; 3, longitudinal 



external aspect :. 



ago, and states that " it is frequently cast up by the waves- 

 on the shores of this island, and it is one of those fruits 

 thrown (also) on the north-west islands of Scotland by 

 the currents and seas." None of these fruits appear to- 

 have germinated in Jamaica, for the palm, which is a 

 most striking object, with entire leaves, could not fail to 

 be noticed if it existed there. The fruits are known at 

 Jamaica as sea-cocoaniifs, while at Barbados they are 

 called sea-apples. Other South American fruits found 

 drifted at Jamaica were Carapa guianeiisis, Dimorphandra 

 mora^ and Asirocaryum sp. The collection of drift-fruits^ 

 although by no means a new subject, is one which has 

 not received the attention it deserves. Since the publica- 

 tion of the "Botany of the Challenger^' the active part 

 played by the sea in the dispersion of plants has been 

 more fully recognized. Mr. Hemsley has given a 

 summary of results, in which many interesting facts 

 have been brought to light, and doubtless observers in 

 numerous parts of the world are now engaged in making 

 observations. 



