368 



♦ KNOWLEDGE • 



[May 1, 1885. 



time for the fibres to accommodate themselves to the fresh 

 light waves. " All good colour," Ruskin tells us, " is 

 gradated. A blush-rose (or, better still, a blush itself, is 

 the type of rightness in arrangement of pure hue." The 

 painfulness of very crude contrast probably arises partly 

 from over-stimulation of too sensitive a structure, and 

 partly from the too sudden withdrawal of the stimulus from 

 one set of structures, and the equally sudden excitement of 

 another set. 



It will now be well understood that the enjoyment of 

 gradated Urjht and shade is due to a gradual passing of 

 action into rest and rest into action. The disagreeable 

 effect of flickering must be noticed, because the explana- 

 tion offered by Mr. Grant Allen seems at least inadequate. 

 He says, " All intermittent and jerky stimulation of the 

 optic nerve is unpleasant, because it attacks the fibres 

 jnat as they are freshly repaired, and therefore most 

 sensitive. To this class belong the unpleasant effects 

 of flickering and unsteady lights." This may be 

 partly correct, but if it be wholly correct flickering 

 ought to be actually pleasant, when the light is very 

 faint. For supposing that the brightest light of the flicker 

 is only strong enough to stimulate the freshly-repaired 

 fibre, just as much as ordinary sunshine would stimulate 

 a fibre somewhat blunted by exposure to daylight, the 

 resulting sensation ought to resemble that produced by the 

 goft radiance of a spring morning. Certainly it ought not 

 to be more painful than the stimulus given by green to 

 " freshly-repaired " fibres which have stood nearly idle 

 while their neighbours have been receiving waves of red. 



It seems that we must again take into consideration the 

 principle of smoothness and continuity. When we look 

 at a flickering candle or lamp, the pupil, contracted at any 

 particular moment to the degree required by the intensity 

 of the light which it receives, is suddenly called upon to 

 dilate, in order to admit a greater amount of fainter light. 

 It has not quite adapted itself to its new circumstances 

 when a stronger stimulus obliges it again to contract, and 

 before it has quite accomplished this task, again it is 

 required to dilate. If the illumination be feeble, even at 

 its brightest, the amount of light admitted at any time 

 might not be sufficient to injure the eye, though the pupil 

 should remain dilated. But, as a general rule, contraction 

 is necessary when there is an increase of brightness, and 

 the automatic instinct acts according to this rule, making 

 no exception for particular cases. No doubt the rods and 

 cones of the retina have their share in the discomfort, and 

 ire in their own manner continually trying and failing to 

 adjust themselves to the change of circumstances. 



In speaking of the esthetic use of black, Mr. Allen 

 remarks, " In short, the real pleasure in every instance is 

 derived from the positive light, and the blackness only acts 

 as a repairer or non-stimulant." Why, then, does not the 

 positive light produce the disagreeable sensation of a 

 flicker? On our own hypothesis we can answer that when 

 the light occurs in large masses there is generally a sensible 

 gradatkra from light to dark ; when it occurs in small 

 bright points, the brightness and darkness are perceived 

 iimuUaneovMy, not successively. If, however, the points 

 be too near together, so that the eye ranges from one 

 to the other, the effect is bad. A black dress, thickly 

 sprinkled with well-defined white spots, is almost unbear- 

 ably dazzling. 



{To he continued.) 



MIGRATION OF BLOWING VIPERS. 



Bv C. Few Seiss. 



A FEW seasons ago, a narrow sandy island on the 

 coast of New Jersey was overrun with countless 

 numbers of the common toad {Bufo lentiyinosv.s Ameri- 

 camis). The toad is generally of crepuscular habits, 

 except during cloudy and rainy weather, but here they 

 were met with, out in search of food, at all hours of the 

 day, even beneath the hot glare of the noonday sun. It 

 may be that, had they all waited until the cool of the 

 evening to hunt for their insect prey, many of the weaker 

 and less active toads would have been supperless. So, by 

 hunting both by day and night, they were able to secure 

 both diurnal and nocturnal insects. Over two hundred 

 toads were counted in a short stroll between four and five 

 o'clock in the afternoon of a July day. At this period 

 there were no snakes of any kind to be met with on the 

 island. That a few did exist I do not doubt, but they were 

 not observed. 



Now, this narrow island is separated from the mainland 

 by a small bay, or thoroughfare, which is, perhaps, over a 

 quarter of a mile wide at its narrowest portion. The vege- 

 tation on the island consisted of little else than rank grass, 

 stunted cedars, and pines. 



A " Begixner-s' Star Atlas," by the Eev. I. E. Eapin, with an 

 introdnction by ilr. .J. A. Wescwood Oliver, is in the press, and will 

 be published shortly by Messrs. Sonnecschein & Co. 



In the season following the one above noted, the toads 

 were again innumerable, but, what was startling, " blowing 

 vipers" (Ileterodon plafyrhhu/.s) were numerous also. They 

 were observed in nearly every part of the island, and were 

 seen pursuing, capturing, and swallowing the toads, as 

 though bent on their extermination. Sometimes a toad 

 would endeavour to escape by quickly burrowing into the 

 sand, but the snake, having marked the spot where the toad 

 disappeared, would force its head, with shovel-like snout, 

 into the sand, seize the unfortunate toad, drag it from its 

 hiding-place, and swallow it (see illustration). 



W^hat was the cause of tliis sudden appearance and num- 

 ber of snakes 1 They made their appearance in early 

 summer, when the young Ileterodons were not yet out of 

 the egg, and it requires several months of growth before 

 they are capable of mastering an average-sized toad. Did 



