February 9, 1893J 



NATURE 



;55 



it approaches the border of the pupil, and especially as it passes 

 on to the iris, it is seen to be accompanied by a small, pale, but 

 well-defined image, which always lies between the first image 

 and the centre of the pupil, the distance between them decreas- 

 ing as they move towards the centre, where they finally coincide. 

 By means of the ophthalmophakometer— an instrument consist- 

 ing of three incandescent lamps and a telescope arranged on an 

 arc of 86 cm. radius — it was found possible to measure the radii 

 of curvature of all the reflectin:j surfaces. The foci of the two 

 reflecting surfaces of the cornea were found to coincide, a fact 

 which accounts for the coincidence of the two corresponding 

 images at the centre of the pupil, and for Helmholtz's failure of 

 finding the fainter one. 



It is evident that since the light reflected from the successive 

 surfaces does not fall upon the retina, it is lost for visual pur- 

 poses. But a comparison of the percentages of loss in the case 

 of the eye, and in that of a simple lens tells greatly in favour of 

 the former as an optical instrument. In the eye the percentage 

 of useful light is 97, in a simple lens 92, and in a compound 

 optical instrument correspondingly less. But the light reflected 

 by any of the internal surfaces is also liable to be reflected back 

 into the eye or the optical instrument, with the effect of super- 

 imposing a more or less faint patch of light upon the image on 

 the retina. This is termed the noxious light {lumiere nuisible) 

 by M. Tcherning. In a simple lens this amounts to ,V per cent., 

 whilst in the eye it is as low as 0002 per cent. But faint as it 

 is, it is capable of giving rise to two light impressions due to 

 double reflection, one at least of which has been actually 

 observed in the human eye. " The easiest way of observing it," 

 says M. Tcherning, " is to look straight forwards in a dark room, 

 holding a lighted candle in the hand about 20 cm. from the line 

 of vision. On moving the candle gently from side to side a pale 

 image of the flame is seen on the opposite side of the line of 

 vision, distinct enough to show that it is inverted ; it moves 

 symmetrically to the candle with respect to the line of vision. 

 The rays which form this image have undergone, besides several 

 refractions, two reflections, one at the posterior surface of the 

 crystalline and another at the front surface of the cornea." 

 Another image was expected to be formed by a similar reflec- 

 tion at the anterior surface of the crystalline. It was found in 

 an artificial eye, but not in the human sense-organ. However, 

 an easy calculation of the optical system of the eye explains this 

 circumstance. The foc>is of the reflected rays is very near the 

 crystalline lens itself, so that they must he much dispersed by 

 the time they reach the retina. To enable the image to be 

 formed on the retina, the object would have to lie between the 

 cornea and the crystalline, but on attempting to form a luminous 

 point at that place by optical means it is found that the "useful 

 rays " fill the eye to such an extent as to render everything else 

 invisible. 



It is found that different eyes differ in their capacity of seeing 

 the first of the two additional subjective images. Short-sighted 

 people find it very indistinct unless the candle is held close to 

 the eye, or convex glasses are used. As the maker of optical 

 instruments utilises the accessory images for testing the degree 

 of polish and the accurate centreing of the lenses, so the 

 physician is enabled to make valuable inferences from them as 

 to the structure and condition of the eye he is examining, and 

 the additional images discovered by M. Tcherning appear to 

 be o f considerable physiological importance. E. E. F. d'A. 



A BOTANIST'S VACATION IN THE 

 HAWAIIAN ISLANDS. 

 COME weeks ago we reprinted from the Botanical Gazette 

 ^ (Indiana) a part of the first instalment of Prof. D, H. 

 Campbell's interesting account of his vacation in the Hawaiian 

 Islands. The following is the chief portion of the second and 

 concluding instalment, published in the January number :— 



Beside visiting the isle of Oahu, I made short trips to the 

 islands of Hawaii and Kauai. The former, the largest of the 

 group, and the only one where volcanic action is still going 

 on, is reached by steamer in about thirty-six hours from Hon- 

 olulu. On the way, the islands of Molokai, Lanai, and Maui 

 are passed. The first, a barren-looking and forbidding spot, 

 is the location of the leper settlement, to which all persons 

 afflicted with leprosy are sent as soon as their condition becomes 

 known. 



NO. I 2 15, VOL. 47] 



Maui, the largest of the islands next to Hawaii, consists of 

 two portions connected by a narrow isthmus. The whole 

 eastern half is nothing more nor less than the body of an im- 

 mense extinct volcano, ten thousand feet high, and with a 

 crater nearly ten miles across. The other end of the island is 

 an older formation. This island is said to be very interesting 

 botanically ; but, unfortunately, my time did not permit me to 

 visit it. 



Very soon after sighting Maui, the three great mountain 

 masses of Hawaii began to loom up. The day was clear, and 

 the whole formation of the island became visible. It consists 

 of three great volcanic cones, of which only one is now active. 

 The highest summit, Mauna Kea, is nearly 14,000 feet above 

 the level of the sea ; the next, Mauna Loa, lacks but a few 

 hundred feet of this ; yet so great is the breadth of these 

 masses that one fails to realise their immense height. Our 

 first landing was at Mahukona, on the leeward side of the 

 island, a most forlorn expanse of bare lava with scarcely a 

 trace of vegetation, except a few unhappy-looking algaroba 

 trees planted about the straggling buildings that constituted the 

 hamlet. 



We lay all day at this inhospitable station, not getting away 

 until evening. A beautiful sunset and a fine glimpse of the 

 peak of Mauna Kea glowing with the last rays of the sun, form 

 my most pleasant recollections of this desolate place. 



What a change the next morning ! On awakening we found 

 ourselves entering the harbour of Hilo. Here everything is 

 as green as can be imagined, and luxuriant vegetation comes 

 down to the very ocean's edge. The town is built on a bay 

 fringed with cocoa-nut trees and embowered in a wealth of 

 tropical vegetation. Owing to the great annual rainfall 

 (about l8o inches), as well as to the fact that Hawaii is the 

 most southerly of the islands, the vegetation here is the most 

 luxuriant and tropical found in the whole group. I remained in 

 Hilo for six days and collected some most interesting specimens. 

 Through the kindness of Mr. Hitchcock of Hilo, I was enabled 

 to spend the night at his camp in the woods near the town, and 

 the greater part of two days collecting in the vicinity. The 

 forest here is most interesting. Mr. Hitchcock was starting a 

 coffee plantation and has cut trails through the woods in several 

 directions, so that collecting was very convenient. There is 

 great danger of ^losing one's self in these woods where there are 

 no trails, as much of the forest is an almost impassable jungle. 

 In these moist forests ferns and mosses luxuriate, and every 

 trunk and log is closely draped with those beautiful growths. 

 Flowers are almost entirely wanting, a fact repeatedly observed 

 by collectors in tropical forests. I saw here fully developed 

 specimens of tree-ferns. The finest of these were species of 

 Cibotium. Many had trunks from fifteen to twenty feet high, 

 and some must have been fully thirty. The most beautiful were 

 some with trunks ten to fifteen feet high, as these were more 

 symmetrical and had finer fronds than the taller ones. I meas- 

 ured the leaves of one that had fallen over, and roughly esti- 

 mated the length as eighteen feet. I have no doubt that 

 specimens fully twenty feet long could be found. These giant 

 fronds, arching high over one's head as one rides on horseback 

 under them, present a sight at once unique and beautiful. 

 Growing upon the trunks of these ferns were many epiphytic 

 species, the most peculiar of which was Ophioglossum pendiilum, 

 with long strap-shaped leaves, a foot or two long, and a spike 

 of sporangia sometimes six inches long. Exquisite species of 

 Hymenophyllum and Trichomanes, the most ethereal of all the 

 fern tribes, with almost transparent, filmy leaves, were common, 

 sometimes completely enveloping the trunks of the trees. Of the 

 terrestrial ferns, which abounded everywhere, two were espe- 

 cially notable as; representing groups unknown in the United 

 States, One of these, Gleichenia dichotoma, forms extensive 

 thickets on the borders of the forest, and in the Hilo district 

 extends down almost to the sea-level. The other, Marattia 

 Donglasii, a very large fern with leaves eight to ten feet long in 

 well grown specimens, has fleshy dark green leaves, and thick 

 stipules sheathing the base of the leaf-stalks. Several species 

 of Lycopodium and Selaginella were common, and a good 

 variety of mosses and liverworts. In these forests wild bananas 

 are common, and most magnificent plants they are. Sheltered 

 from the wind, the superb great leaves develop to their full size, 

 without being lorn in the least, and the whole plant is a study 

 of beautiful form and colour. 



Coffee is being extensively planted in this region as well as 

 upon the lee side of the island, and as the quality of the berry 



