138 ANNUAL OF SCIENTIFIC DISCOVERY. 



X" 



When I used a broad red stripe between two green ones, the change of 

 color was confined to the borders of the red, extending to a breadth of 

 about a quarter of an inch, a bright stripe appearing on the upper side 

 of the red when the figure was depressed, and on the lower side when 

 the figure was elevated. No change of appearance could be observed 

 in the intermediate red. Hence, when the red stripe is only a half 

 inch in breadth, there is the appearance of a red wave transferred from 

 one edge to the opposite, at each motion of the card. If the red 

 stripe does not exceed a quarter of an inch in breadth, the effect is 

 much impaired, and if the stripe be reduced to less than one tenth of 

 an inch, the wave-like appearance ceases entirely, and the red stripe 

 appears constantly of a very pale red. 



The following, then, is my opinion of the origin of this deep red 

 wave : The red color appears to excite the retina more powerfully than 

 the green, and its impression is more durable. When, therefore, I 

 place a red stripe upon a green ground, and agitate it, the effect of the 

 green ground is confined to a narrow margin of the red stripe, not gener- 

 ally exceeding a quarter of an inch in breadth. Consequently, if the 

 red stripe is broad, say two or three inches, no peculiar effect is pro- 

 duced upon the central part ; but a band of pale red, about a quarter 

 of an inch in extent, is seen on the two opposite margins alternately. 

 If the breadth of the red stripe does not exceed the tenth of an inch, its 

 light is constantly blended with that of the surrounding green ; that is, 

 it appears constantly of a light shade, and there is no appearance of 

 a dark wave passing over it. When the breadth of the stripe is about 

 half an inch, the red and green colors are made to combine on the 

 opposite margins alternately ; the dark and the light bands succeed 

 each other on the same part of the stripe, and the appearance is that 

 of a wave-like motion. Thus, the two well known principles, first, 

 of the continuance of impressions on the retina, and, second, that 

 complementary colors combine to produce white light, appear to explain 

 the essential circumstances of the phenomena in question. 



TELOCITY OF LIGHT. 



ON the subject of the velocity of light, and the probability that it 

 requires a certain time for its propagation, we find the earliest view 

 expressed by Francis Bacon, in the second book of the Novum Or- 

 ganuni. He speaks of the time required by a ray of light to traverse 

 the immensity of space, and throws out the question whether the stars 

 still exist which we now see sparkle. One is astonished at finding so 

 happy a conjecture in a work whose celebrated author was so far 

 below some of his contemporaries in mathematical, astronomical and 

 physical knowledge. The velocity of the reflected solar light was 

 measured by Komer, November, 1675, by comparison of the times of 

 occultation of Jupiter's satellites, and the velocity of the direct light 

 of the fixed stars by Bradlcy's great discovery of the aberration of 

 light, made in the autumn of 1727. In very recent times a third 

 method of measurement has been proposed by Arago, by the phenome- 

 non of the light of a variable star ; for example, Algol in Perseus. We 



