*594 



SCIENCE. 



Science, observed in day light by the sportsman, when he en- 

 Cuiipsities devours to mark, upon the monotonous heath, the par- 

 n> ticular spot where moor-game has alighted. Availing 

 s "V"' himself of the slightest difference of tint in the adjacent 

 heath, he keeps his eye steadily fixed upon it as he ad- 

 vances ; but whenever the contrast of illumination is 

 feeble, he invariably loses sight of his mark, and if the 

 retina is capable of again taking it up, it is only to lose 

 it again. 



Mr. Herschel and Mr. South, see the Phil Trans. 

 1824, Part iii. p. 15. have recently observed a very 

 curious fact, which has some analogy with the phe- 

 nomena now described. 



" A rather singular method," they remark, " of ob- 

 taining a view, and even a rough measure of the angles 

 of stars, of' the last degree of faintness, has often been 

 resorted to, viz. to direct the eye to another part of the 

 field. In this way, a faint star, in the neighbourhood 

 of a large one, will often become very conspicuous, so as 

 to bear a certain illumination, which will yet totally 

 disappear, as if suddenly blotted out, when the eye is 

 turned full upon it, and so on, appearing and disap- 

 pearing alternately, as often as you please. The lateral 

 portions of the retina, less fatigued by strong lights, 

 and less exhausted by perpetual attention, are probably 

 more sensible to faint impressions than the central 

 ones, which may serve to account for this phenome- 

 non."* . 



As it is with much diffidence that I venture to con- 

 trovert any opinion entertained by Mr. Herschel, I 

 have been at some pains to investigate the subject ex- 

 perimentally. I was, at first, disposed to ascribe the 

 evanescence of the faint star, solely to the same cause 

 as the evanescence of faintly illuminated surfaces, and 

 the reappearance of the star by indirect vision, to the 

 circumstance of the retina recovering its tone, by con- 

 templating another object sufficiently luminous for vi- 

 sion ; but this opinion was not well founded. 



If a given quantity of light, which is unable to af- 

 ford a sustained impression when expanded over a 

 surface, is concentrated into a luminous point, it is 

 still less fitted for the purposes of vision. It then acts 

 upon the retina somewhat in the same way as a sharp 

 point does upon the skin. The luminous point will 

 alternately vanish and reappear ; and if the retina is 

 under the influence of a number of such points, it will 

 be thrown into a state of painful agitation. The same 

 effect is produced by a sharp line of light ; the retina 

 is, in this case, thrown into a state of undulation, so 

 as to produce an infinite number of images parallel to 

 the luminous line ; and when this line is a narrow 

 aperture held near the eye, a sheet of paper, to which 

 it is directed, will appear covered with an infinity of 

 broken serpentine lines parallel to the aperture. When 

 the eye is stedfastly fixt d, for some time, upon the pa- 

 rallel lines which are generally used to represent the 

 sea in maps, the lines will all break into portions of 

 serpentine lines, and red, yellow, green, and blue tints 

 will appear in the interstices of them. 



The evanescence of stars, therefore, of the last de- 



gree of faintness, must be ascribed, both to their de- Science, 

 leterious action upon the retina as points of light, and Curiosities 

 to the insufficiency of their light to maintain a con- in - 

 tinued impression upon that membrane. *"* "Y^* 1 ' 



When the same star is seen by indirect vision, it re- 

 appears with a degree of brightness which it never 

 assumes when seen directly by the eye. When the 

 eye is adjusted to the distinct perception of an object 

 placed in the axis of vision, an object placed out of the 

 axis cannot be seen with the same distinctness, both 

 from the pencils not being accurately converged upon 

 the retina, and from the expansion of the image, which, 

 as we have already described, accompanies indirect 

 vision. A luminous point, therefore, seen indirectly, 

 swells into a disk, and thus loses its sharpness, and 

 acts upon a greater portion of the retina.t In order 

 to determine whether this expansion, and the knage 

 of the luminous point, was the cause of its superior 

 visibility, I turned my eye fall upon a luminous point 

 till it ceased to be visible, and then re-adjusting my 

 eye, so as to swell the point into a circular disk by 

 direct vision, I invariably found that its visibility was 

 i-.uUiitlr increased. If this explanation of the pheno- 

 menon be the correct one, the practical astronomer 

 may, with direct vision, obtain a clearer view of mi- 

 nute and faint stars, either by putting the telescope 

 out of its focus, or by adjusting his eye to nearer ob- 

 jects." 



16. Singular Illusion in examining a Dioramic Picture. 



In examining a dioramic representation of the inside s ' n g u ' r >1- 

 of Rochester cathedral, which produced the finest ef- g"^ n ^ 

 feet from the entire exclusion of all extraneous light and a^aicr'amic 

 of all objects, excepting those on the picture itself; the picture, 

 writer of this article was struck with an appearance of 

 distortion in the perspective, which he ascribed to the 

 canvas not hanging vertically. Upon mentioning this 

 to the gentleman who exhibited the picture, he offered 

 to walk in front of it, and strike its surface with the 

 palm of his hand, to show that the canvas was freely 

 suspended. Upon doing this, a very remarkable de- 

 ception took place. As his hand passed along, it gra- 

 dually became larger and larger, till it reached the 

 middle, when it became enormously large. It then di- 

 minished till it reached the other end of the can- 

 vas. 



As the hand moved towards the middle of the pic- 

 ture, it touched parts of the picture more and more 

 remote from the eye of the observer, and consequently 

 the mind referred the hand, and the object in contact 

 with it to the same remote distance, and consequently 

 gave it an apparent magnitude such as a body of it* 

 size would have had at the distance of the part of the 

 picture which it covered. 



We have seen an analogous illusion when viewing 

 the mosaic pavement of St. Paul's from the inside of 

 the cupola. The lozenges had a certain apparent 

 magnitude when seen alone, which of course was small. 

 When a person, however, passed over the pavement, 

 our knowledge of his size furnished us with a scale 



Edinburgh Journal of Science, Vol. I. p. 80.) Hence, if an object presents itself within that distance, the observer must see it with a de- 

 gree of indistinctness which cannot fail to surprise him, especially as all distant objects, particularly those seen against the sky, will appear 

 to him with their usual sharpness of outline. 



* A similar fact wth regard to the satellites of Saturn was noticed by some of the astronomers in the Royal Observatory of Paris. 



f The eye is not capable of observing the colours of luminous points seen indirectly. A blue luminous point, for example, appears 

 nearly white, and so do points of any other colour. 



