66 



NATURE 



[November 21, 1895 



in the Kew Herbarium, has been good enough to undertake 

 a critical examination of Criiger's drawings, and compare them 

 carefully with the description and plates of the species in the 

 "Flora Brasiliensis." The result of the investigation is con- 

 tained in the following note :— 



"Criiger's drawings oi Sacoglottis, Mart. ' Cojon de Burro,' 

 October 13, 1861, agree perfectly with Sacoglottis amazonica. 

 Mart. (' Flora Brasiliensis,' vol. xii. pt. 2, p. 449, tab. xcv.) 

 The analyses are very carefully done. The sketch of the base 

 of the leaf, for instance, shows the characteristic two glands 

 which had been overlooked by Martius as well as Urban. As 

 the fruit has not been previously described, it appears desirable 

 to give a description of it drawn up from Criiger's drawings, as 

 well as from the several specimens in the Kew Museum. 



"Fructus subdrupaceus, ellipsoideus, \\,-\\ poll, longus, 

 \-\\ poll, latus, exocarpio vix i lin. crasso, endocarpio osseo, 

 extus subbuUato, cavernis resina impletis abundante, 5-loculari 

 vel saepius ob ovula loculosque i-4aborta 4-1-loculari ineunte 



IS evident that from one or both of these localities the fruits are 

 carried by the waters of the Gulf Stream into the Caribbean 

 Sea, and either thrown ashore on the West Indian Islands, or 

 carried still further, as in the case of many other similar fruits, 

 across the North Atlantic, and cast on the shores of Western 

 K»rope. D. Morris. 



Fruit of Sacoglottis amnzonica, Mart, (after Criiger). 



germinatione valvis 5-trigonis ab axi 5-alata semina inter alas 

 in loculis late apertis exhibente sedentibus dehiscente. Semina 

 cylindrica, pollicaria, testa tenui nigro-brunnea, albumine carnoso, 

 embryonecentrali, cotyledonibus lineari-oblongis planis, radicula 

 brevi supera. 



" The breaking up of the fruit, as described above, takes place 

 also in water-worn specimens, as shown in the fruit collected by 

 Dr. Nicholls. Sacoglottis amazoiiica was previously known only 

 from Teflfe or Egas, on the right bank of the middle Amazon, and 

 from the banks of the Tagipurii, a channel in the delta of the 

 Amazon, where it was collected by Martius. It is recorded from 

 St. Vincent, on the authority of Guilding. Specimens from the 

 latter are in the Kew Herbarium, but whether from wild or 

 cultivated plants is not stated. It is also not certain whether 

 they did not come, as many of Guilding's specimens, from 

 Trinidad."— O. Stai>k. 



Summary. 

 The story of this interesting drift-fruit is now told. The re- 

 cord of its occurrence has been traced from the year 1605, when 

 it was first figured and described by Clusius, down to 1764, when 

 it was redrawn by Petiver. For about one hundred and fifty 

 years it was successively described by Clusius, Jonston, J. 

 Bauhin, Hans Sloane, and Petiver. From 1764 until 1884, a 

 period of one hundred and twenty years, it appears to have been 

 entirely overlooked. It was, however, once more brought into 

 notice in the latter year, and drifted specimens were obtained 

 within a short period from Jamaica, the South of England, 

 Barbados, the Grenadines (between St. Vincent and Grenada), 

 and Trinidad. The specimen from the latter island was accom- 

 panied by careful drawings made by Criiger in 1861, giving par- 

 ticulars not only of the fruit itself, but also of the leaves and 

 flowers. These when carefully compared with the description 

 and plate in the "Flora Brasiliensis," and with specimens in 

 the Kew Herbarium, left no doubt that the plant yielding the 

 Jamaica drift-fruit is Sacoglottis amazonica. Mart. This grows 

 very sparingly in the south-eastern portion of Trinidad, where 

 it was collected by Criiger, but is more abundant in the delta of 

 the Amazon, where it was collected by Martius and others. It 



NO. 1360, VOL. 53] 



THE PERCEPTION OF LIGHTS 

 AT a former anniversary I brought before the members of the 

 -'*• Institute the subject of the luminiferous ether. It is one of 

 great and growing interest. I mentioned on that occasion how 

 discoveries of very recent date have led us to attribute con- 

 tinually increasing importance, and a widening range of 

 function, to that medium — substance can I call it ? — the existence 

 of which was originally assumed as a hypothesis in order to 

 account for the phenomena of light. It is in connection with 

 this last aspect that it relates to what I propose to bring 

 before you to-day. 



The wonderful sense of sight, which, to use an expression of 

 Sir John Herschel's, confers upon us to some extent the character 

 of ubiquity, requires two things : in the first place, some means 

 by which those distant bodies which we see are able to affect our 

 own neighbourhood ; in the second place, some provision in our 

 own bodies for receiving that influence, and transmitting some 

 sensation to the conscious being. 



In my former address I considered the first of these two 

 subjects ; to-day I mean to confine myself to the second. This 

 second, even by itself, is, however, far too wide for a single 

 address ; selection of some kind is imperatively demanded. 

 Moreover, there are some parts which are accurately known, and 

 may even be made the object of mathematical calculation, while 

 there are others which not merely lie beyond our existing 

 knowledge, but beyond any that we can hope to attain to, at 

 least in this life. Wonderful as is the construction of the eye in 

 all its parts, so far as relates to the formation of images on the 

 retina it acts simply like an optical instrument, like a telescope 

 or microscope, or, more correctly, like the objective of such an 

 instrument, and we may apply our mathematics to tracing the 

 course of the rays through it. On the other hand, even if we 

 knew accurately — which we do not— the nature of the effect 

 which the external agent produces on the ultimate structure of 

 our bodies, there would still remain, shrouded in impenetrable 

 mystery, the nature of the process by which some change in the 

 bodily organism causes a sensation to the conscious being. 



Between these two extremes lies a region which has been to 

 some extent explored, and in which a gradual and perhaps at 

 last a very substantial increase to our existing scientific knowledge 

 may be looked upon as probable. The investigation of this 

 region possesses the . keen interest which belongs to the dis- 

 covery of new truths, and the addition thereby made to the stock 

 of human knowledge. It is to this borderland lying between the 

 well known and the unknown, and to certain parts of the 

 structure of the eye having relation to it, that I would for a 

 short time direct your attention to-day. 



As I have already intimated, I propose to pass by entirely the 

 functions of the eye acting as a simple optical instrument in 

 forming images on the retina. The explanation of that may be 

 found in all the ordinary text-books, and I will not weary you 

 by repeating what is there to be found, and which is generally 

 familiarly known. 



The phenomena of vision show that distinctness of vision is 

 dependent somehow or other in the first instance on the formation 

 of distinct images of external objects on the retina. In that 

 formation, as I have said, the transparent portion of the eye, the 

 cornea, the aqueous humour, the crystalline lens and vitreous 

 humour, plays the part of a lens in an optical instrument. I 

 have said the " formation of the images on the retina"; but 

 the retina is not a mere surface, it has a certain amount of thick- 

 ness, although it is, on the whole, very thin. We may further 

 inquire on what part of the retina, considered at different depths 

 from the place where it first commences, on which of the various 

 layers into which histologists have divided it, is it that we have 

 reason to think that light first acts on the organism of our bodies 

 in such a manner as ultimately to give us the sensation of vision? 

 I have said that the retina, as a whole, though very thin, is 

 not a mere surface. If we go from the centre of the eye-ball 



1 Presidential Address delivered at the Victoria Institute by Sir G. G. 

 Stokes, F.R.S. 



