Sept. 5, 1884.] 



♦ KNOWLEDGE * 



199 



and specks will often be seen floating about,'seemingly in 

 the heavens, and travelling slowly downwards. When these 

 assume the appearance of minute transparent spheres, they 

 become a source of annoyance to the observer of small 

 stars in daylight, whose appearance they simulate. They 

 are really extremely minute specks floating in the vitreous 

 humour (Fig. 25). Or the very curious and instructive 

 experiment devised by Purkinje may be tried. It is thus 

 performed : You take a lighted candle into an otherwise 

 totally darkened room ; shut the left eye, and hold the 

 candle as close as you can to the right eye (say three 

 inches from it), to the right and rather in front, so as to 

 light up the retina strongly. Keeping the right eye steadily 

 fixed on the opposite wall, you move the candle about into 

 various positions. Now, the glare of the light will soon 

 cause the field to appear dark, and then, as the gentle 

 motion of the candle continues, suddenly there starts 

 into view on the wall a strange ghostly kind of 

 branching net-work, or thing like the upper part 

 of an oak-tree when the leaves are ofT in winter. 

 This is really an image of the blood-vessels of the 

 retina. They enter the eye at the same point as the optic 

 nerve, and spread out over the sensitive surface of the 

 retina. The candle casts their shadows upon it, and they 

 are projected, and seem to exist on the opposite wall. The 

 words " sensitive surface of the retina," which we have just 

 written, suggest to us to remark that the retina is by no 

 means equally sensitive over its whole area ; in fact, that, 

 as we shall immediately see, there is one spot in it which 

 is absolutely blind or insensitive to light, while in another 

 the greatest exaltation of visual perception obtains. In the 

 axis of the eye is situated what is known as the central 

 spot, a thinning or actual depression of the retina. If we 

 look out over an expanse of country without moving the 

 head or eyes, we shall observe certain objects immediately 

 in front of us, sharply and brilliantly defined ; but if we 

 attend a little, we shall note that, as we recede from them, 

 either to the right or the left, upwards or downwards, every- 

 thing becomes gradually less distinct, and with a more im- 

 perfect outline, so that at any considerable angular distance 

 from the point of sharpest vision things become blurred 

 and amorphous. In practice, and unconsciously, we shift 

 the eyes, and so successively take in the various details of 

 the landscape, which we fancy we see altogether ; albeit, 

 as we have explained, we do nothing of the sort. The 

 central spot, of which we have spoken above, is the seat of 

 the sharpest vision. On the other hand, the place where 

 the optic nerve enters the eye-ball is utterly insensitive to 

 light, and hence is known as the blind spot Fig. 20 will 

 enable the reader to verify its existence in his own eye for 

 himself. Let him shut the left eye and steadily regard the 

 little cross with his right one, say at a foot or eighteen 

 inches distance. He will see both the cross itself and the 

 dark disc perfectly. Now, let him slowly bring the page 

 nearer to Ins face, and at a certain point the circular disc 

 will vanish and the paper appear white. At this instant 

 the image of the disc has fallen upon the blind spot of his 

 right eye. An he continues to approximate the paper to 

 his eye, the disc will once again reappear, its image now 

 falling upon another sensitive part of the retina. 



(To he continued.) 



Thirty-kive years ago the ratio of railway passengers killed in 

 the United Kingdom from causes heyond their own control was 

 one in 4,782,000, while last year it was one in 61,810,000. In 18S3 

 only eleven persons were killed from causes beyond their control. 

 As compared with this, upwards of 200 persons are annually killed 

 in the streets of London. 



HOW AMERICAN CARP ARE 

 DESTROYED. 



THERE is a little plant, common enough in our ponds, 

 and known as the bladder-wort, which has suddenly 

 sprung into importance for breeders of carp. The bladder- 

 wort (genus Utrimlaria) is a rootless plant fond of still 

 water, and usually found floating half in and half out of 

 water, the branching and stem-like leaves forming the 

 submerged float from which rises the flower stem. To the 

 leaves are attached curiously insect-like bladders filled 

 with water, and varying in size in the difiisrent species, 

 reaching at times a diameter of one-fifth of an inch. 



It was formerly, and with much probability, supposed 

 that these bladders served the purpose of floats ; for until 

 a few years ago it was taken for granted that air and not 

 water filled them. It is now known, however, that the 

 bladders serve a more useful purpose than merely to keep 

 the head of the plant above water ; they are the digestive 

 organs of the Utricularia, and at the same time are so 

 constructed as to form a very ingenious but extremely- 

 simple trap for catching food. It is into these bladders 



A. Bladder (with small fish caught). B. Longitudinal section of 

 bladder. C. Branch showing leaves and bladder. 



that thousands of carp eggs find their uuwitting way, to- 

 gether with many insects, Crustacea, and other tiny objects, 

 both animate and inanimate. 



It is only recently that the Ulricularia has been accused 

 of destroying carp eggs, but for nearly thirty years it has 



