40 



KNOWLEDGE. 



[Pebru.\ky 1, 1894. 



the rays will now, of course, converge behind the retina, 

 and, as before, a number of ill-defined similar images will 

 be seen, each with its complement of rays. Bearing in 

 mind the direction in which the rays now fall on the 

 retina, it will be seen that the various pencils will combine 

 to form two large pencils with foci lying beyond the 

 images, as in the presbyopic eye. 



An explanation of the cause of these rays would be 

 interesting. J. Walter Browx. 



Belfast, January 16th, 1894. 



[The rays referred to by Mr. Walter Brown are well 

 known. They are produced by the reflection of light from 

 the wet surface of the edge of the eyelid when it partly 

 overlaps the pupil of the eye ; and thus they are only seen, 

 as Mr. Brown remarks, when the eyes are partly closed. 

 He may satisfy himself that the rays are produced by 

 reflection from the edge of the lid by a few simple experi- 

 ments. 



If he entirely closes one eye and partly closes the other 

 when looking at a distant candle, so as to see one upward 

 and one downward ray, he will find that the lower ray is 

 eclipsed if the edge of a visiting card is brought slowly 

 downwards between the eye and the candle, and that the 

 upper ray is eclipsed if the edge of the card be brought 

 between the candle and the eye from below upwards. 



If he obtains the assistance of a friend to watch the 

 operation, he will find that he begins to lose sight of the 

 end of the lower ray at the moment that the shadow of the 

 card reaches the edge of his upper eyelid, and similarly 

 with the upper ray when the shadow of the card reaches 

 the edge of his lower eyelid. On inclining his head to one 

 side, he will find that the rays revolve in proportion as 

 the head is turned round ; and by carefully laying hold of 

 his eyelashes, and stretching or twitching the lid, he may 

 alter the position and form of the rays. 



These pencils of rays are, no doubt, the rays which artists 

 intend to represent by the diverging lines they draw from 

 the picture of a candle, but they do not correspond to the 

 very numerous and fine bright rays which we see diverging 

 from the images of brilliant points of light such as 

 electric arc lights, or the reflection of the sun from the 

 bulb of a thermometer. These finer rays are seen when 

 the eyes are wide open, and they are produced by radial 

 inequalities in the structure of the crystalline lens of the 

 eye — a curious structure grouped into six bundles or 

 sectors, which is well shown in a picture in Helmholtz's 

 " Physiological Optics " (see the I'rench translation, p. 34). 



The eyelids, when closing over the pupil, do not form a 

 slit with parallel edges ; and the two arms of the Y seen 

 by Mr. Brown seem to indicate that while the edges of 

 his upper eyelids are parallel, the edges of his lower eye- 

 lids are inclined to one another. — A. G. Eanyard.] 



ENCICE AXD SEEBEKG. 

 To the Editor of Knowledge. 



Sir, — We have all heard of the complaint of Socrates 

 that Crito confounded him with his corpse by asking him 

 how he wished to be buried. By a stranger, or at any 

 rate more unusual mistake, your readers this month must 

 have thought that I had confounded Encke with his 

 dwelling-place during the early part of his astronomical 

 career. But what I wrote (p. 7, col. 1, line 20) was not 

 "the illustrious Heeberg, an astronomer," but "the 

 illustrious Seeberg astronomer," Encke having been at 

 that place when he determined the orbit of the comet with 

 which his name is indissolubly connected. 



The Seeberg Observatory, founded by Duke Ernest II. 

 in 1791, was situated a few miles from Gotha. Zach and 



Lindenau were successively directors of this establishment, 

 where Encke laboured from 1815 until he was appointed 

 director of the Berlin Observatory in lH2y. The building 

 at Seeberg had fallen into decay, and the instruments were 

 transferred to a new observatory, erected at Gotha, in 

 1857, where Hansen, Encke's successor, remained until 

 his death in 1874. 



Yours faithfully, 

 Blackheath, .Jan. 1st, 1894. W. T. Lynn. 



THE ASTRONOMICAL SOCIETY'S MEDAL. 



The Council of the Eoyal Astronomical Society have 

 this year awarded their gold medal to Mr. S. W. Burnham, 

 who has long been acknowledged &s facile printrps of double 

 star observers. He is not only a very keen-eyed observer, 

 who has detected nearly all the very close and most diflicult 

 double stars that are known, but he has discovered more 

 double stars than Sir William and Sir John Herschel, 

 Wilhelra and Otto Struve, and Dembowski put together, 

 and has made a most exhaustive study ot the history of 

 double star observation, which has enabled him, by a 

 comparison of the observations of all the well-known 

 double star observers with his own observations, to 

 determine with far greater accuracy than was previously 

 possible the forms of the apparent orbits of all the 

 principal binary systems. Sherburne Wesley Burnham 

 was born about 1M40 at Thetford, A'ermont, U.S.A. His 

 first discoveries were made with an excellent six-inch 

 refractor, made by Alvan Clark. Subsequently he worked 

 with the eighteen and a half inch refractor of the Dearborn 

 Observatory, Chicago, and then with the thirty-six inch 

 refractor on Mount Hamilton. He retired from the Lick 

 Observatory some two years ago, but he has now been 

 appointed to the Yerkes Observatory, where he will have 

 ample opportunities of using the forty-inch refractor. 



STINGING INSECTS. -I. 



By E. A. BuTLEB. 



AMONGST the varied powers possessed by insects, 

 there are, perhaps, none that have made so forcible 

 an impression upon the human mind as that of 

 stinging. The intensity of the pain that follows 

 a stmg, contrasted with the insignificance of the 

 being that causes it, and the quietness, suddenness, and 

 unexpectedness of the operation, whereby there is no time 

 left for the person attacked to put himself on his guard, or 

 to prepare for defence, are circumstances well qualified to 

 inspire caution. To facts like these may, no doubt, be 

 attributed the very prevalent feeling of suspicion with 

 which unknown insects are regarded, especially if they 

 have a formidable appearance. In consequence of this 

 widely-difl'used distrust, many insects that are perfectly 

 hai-mless are popularly credited with venomous powers, 

 especially if their shape or general aspect suggests such a 

 property. The prejudice thus created is most difficult to 

 eradicate. It begins in the nursery. Children when quite 

 young are cautioned against creatures that could in no way 

 harm them, and this influence descends from one to the 

 other as part of the inheritance of nursery traditions ; the 

 error is thus perpetuated, and shows at times such extra- 

 ordinary vitality that the strongest assurances of experts 

 are insufficient to dispel the illusion. And yet a very Uttle 

 general knowledge of insect structure is enough, without 

 any close acquaintance with particular species, to enable 

 an ordinary person to judge for himself whether any given 

 insect is a stinger or not. We propose in this paper to 



