863 



THE POPULAR EDUCATOR. 



pocted, are not affected alike by these experiments ; but all see 

 whiteness passing into yellow, orange, and blue, and blue 

 returning back in deep orange, yellow, and white. The 

 restoration of the light is on the whole less satisfactory than 

 its reduction, for when by reduction a deeply intense blue is 

 obtained, the light cannot, to some eyes, be restored slowly 

 enough to prevent a sudden change to deep orange. The colours 

 that succeed each other, as the light is gradually reduced, have 

 none of the accepted relations between any given colour and its 

 complement. The white is not succeeded by thin blackness, 

 the yellow by faint purple, or the orange invariably by blue ; 

 but the different hues do come up in an order that suggests the 

 great probability that what we name colour is only the various 

 affection of the optic nerve by a greater or lesser quantity of light 

 radiating from, a focal point in an imperfect reflector." 



The above experiment was the result of accident. Mr. Eose 

 had been looking upon a white surface lying near a powerful 

 gas-light, when his arm having caught the tap and reduced the 

 light, his attention was drawn to a sudden change from white 

 to red. 



Another experiment of great beauty and interest was also 

 suggested to him by an accidental circumstance. He was 

 observing the effect of flashes of light, intermittent artificial 

 light, on a revolving disc having twelve large circular black 

 spaces ranged equidistantly around the margin. It was broad 

 day, and the window-shutters were closed to exclude the 

 natural light. In the course of the experiment the shutter 

 started open and admitted a little daylight, when the remark- 

 able appearance was presented of twelve blue circular spaces lying 

 upon a zone of bright orange. Mr. Hose regarded this at the 

 time as simply the presentation of a complementary colour 

 under singular conditions that kept it permanently before the 

 eye ; but as leisure afforded him opportunity to repeat the 

 experiment, Le soon began to perceive that he had taken far 

 too limited an I narrow a view. The misconception arose out of 

 a fact connected with the painting of the discs. It was found 

 that lampblack alone would not give the depth and intensity 

 required in the devices, and to remedy the defect a little indigo 

 was added. The circular spaces, to the eye, were certainly 

 intense black and nothing more ; but it was considered that 

 they had a tendency to blueness, and that under the rotation 

 they were reduced to a lighter blue, and drew after them trains 

 of complementary orange, in the same way that a black fly 

 walking across a pane of ground-glass, backed by grey light, 

 is seen to draw a white spectrum after it. 



But this idea was dismissed as soon as it was ascertained 

 that absolute unmixed black produced the same effect, and that 

 the nearer the artificial light approached the intensity of white- 

 ness, the more decided and satisfactory was the result. 



How, then, is this effect to be explained ? Mr. Eose says, 

 " The diffused light of the zone is continually falling upon the 

 eye ; but the intermittent flashes find the negations, or black 

 portions, always in the same areas, and hence from these spaces 

 no part of the flash is reflected, whilst it mingles with and adds 

 to the diffused light in the spaces between the negations. 

 Now the diffused light is, we assume, intense light reduced 

 by distribution to blueness, and in this blueness the negative 

 spaces participate ; but in the rest of the zone the flash brings 

 up the light to such quality in relation to space as is necessary 

 for the presentation of orange. We have more light from diffu- 

 sion a j the outer and inner edges than in the centre of the 

 zone or ring, and hence the light blue at the inner margin and 

 'tho light blue passing into green at the outer margin. This 

 common quality of the zone is shown in the negative spaces. 

 But from the intervals between them there comes the diffused 

 light variously affected by the flash, and conveying the graduated 

 tints of orange." 



This explanation of the effect, Mr. Eose thinks, will appear 

 reasonable if the conditions of the action are thoughtfully con- 

 sidered. Eight circular spaces of intense and absolute black- 

 ness produce under rotation and by persistence a nebulous 

 ring. 



" If," he says, "this is to be viewed as a mixture of light and 

 shadow, or of black and white, we cannot explain the manner of 

 its affection by the intermittent light, which shows the apparently 

 stationary negations as blue, and the remainder of the zone as 

 orange. But if we regard the black spaces as utter absence 

 of light, reducing the quantity of light for distribution over 



the zone, but giving it no quality by admixture, all difficulty 

 is at an end. A quantity of light is then understood to be 

 diffused over a certain space, whence it comes modified to blue- 

 ness, and when this reduced light receives the impression of 

 the flash it is increased in relation to surface and raised to 

 orange." 



In the "Edinburgh Journal of Science," Mr. Smith has 

 described a very curious instance of the change of white light 

 into complementary tints. In his directions for the performance 

 of this experiment the operator is directed to hold a strip of 

 white cardboard upright, about twelve inches from the eyes. 

 The card may be six inches long and a quarter of an inch wide. 

 If the eyes are now fixed upon some object at a distance of ten 

 or twelve feet behind it, so that the card becomes doubled, and 

 a lighted candle is now placed close to the right eye and shaded 

 from the left one, the latter will see the white strip of card 

 green, whilst the former will appreciate the complementary 

 colour, or red. On changing the candle so that the light falls 

 upon the left eye, the phenomena are reversed. Does this show 

 that artificial light generally presents an excess of the red waves 

 of light, and that the green is the inevitable result obtained by 

 first exciting the eye with a reddish light ? 



We cannot better conclude these papers on the persistence of 

 vision and its illusions than by giving the reader a general 

 summary of the effects : 



1. Persistence. The retention of an image by the eye, not 

 for an absolute instant, but for an interval an interval suffi- 

 cient for an object to pass over a succession of points, in all 

 of which it will be apprehended by the eye at the same 

 instant. 



Illustration. A lighted stick moved rapidly in a circle 

 presents a ring of light, because tho eye retains an impression 

 of the light at any given point until the stick has returned to 

 the same point again. 



2. Simple Persistence presents only illusions of the simplest 

 character, as the commingling of the elements of white light, the 

 composition of colour, etc. 



3. Persistence under Condition of Interrupted Vision offers 

 an indefinite variety of illusions depending on the fact that a 

 disc in rapid revolution, presenting the points on its circum- 

 ference only for an instant to the eye, is virtually stationary ; 

 and any object situated in those points is distinctly seen, by 

 reason of its making no sensible advance during the exceed- 

 ingly brief interval of its apparitions. 



4. Disc Action. The illusions of persistence are presented 

 under various arrangements of disc action, in which discs 

 revolving with different degrees of velocity, and bearing multi- 

 form devices, impress the eye with a number of images at 

 virtually the same instant. 



5. Single Disc Action. Tolerably well known in its appli- 

 cation to ordinary optical experiments, and as the vehicle for 

 the illusions presented in the thaumatrope, etc. The single 

 action has this advantage in connection with the thaumatrope 

 and kindred devices, that it shows true form, and does not make 

 anamorphoses or distorted figures, in one point of view con- 

 fused, in another exact and regular. 



6. Double Disc Action. This action produces, under certain 

 arrangements, almost an unlimited variety of illusions. The 

 double-disc movement (as arranged by Mr. Eose) consists of 

 two wheels, one of which receives a disc bearing the devices, 

 and the other a black disc perforated with a number of slots or 

 slits. The wheels revolve in contrary directions, and their 

 relative velocities can be varied at pleasure within certain limi- 

 tations. So far as the illusions of persistence aim at no higher 

 purpose than the representation of a horse leaping a gate, etc., 

 the double action is less valuable than the single, because the 

 motion of the wheels being contrary, the diameter of the figuros 

 is determined by the angular motion of the slot, and thus the 

 normal figure of the devices is altered. For example : If 

 the wheels move with equal velocities the diameter is reduced 

 one-half, and a circle becomes an ellipse. The figure is further- 

 altered by any variation in the relative velocities of the two 

 wheels ; but if in these illusions we aim at something higher 

 than a mere optical toy, the double-disc action will rise in our 

 estimation, since it presents the most interesting illustrations of 

 recondite optical principles, and also examples of compound 

 motion, multiplication, involution, and combinations of the 

 most pleasing and attractive character. 



