Not. 18, 1881.] 



KNOWLEDGE 



57 



fore, it is by to means certain that this energy should bo wasted, 

 although it fails to (i.e., is not designed to) form lieat on this littlo 

 globe. — CoGiio. 



A CLEVER SEA-GULL. 



[22] — During a rec n- passage over the North Sea, a flock of 

 sea-gulls followed the steamer for many miles. At last I noticed 

 that one of them, a remarkably fine bird, had, by some chance, got 

 an angler's line attached to its wing. The poor thing Hew about 

 the rigging, its companions, meanwhile, uttering loud cries. After 

 great cawing, the bird flew quickly towards the ship, dashed round 

 one of the ropes several times, and ultimately flew off, leaving the 

 line twisted round the rope. Was this what Mr. Herbert Spencer 

 wonid call " reason in inferior animals ? " 



Hoping to see your journal succeed, as it deserves to do, 

 I remain, yours, 



Geo. B. Eraser. 



Helcnsburg, November 5, 18S1. 



A LCN.UI ILLUSION. 



[23] — With reference to geometrical illasions, I am sure that 

 many persons must have noticed the following, and yet my atten- 

 tion has never been drawn thereto either in conversation or in 

 print : — Tlio crescent moon being Diana's bow, we shall give the 

 name " sagitta" to the line drawn from the middle of the invisible 

 bow-string connecting the two horns of the moon to the middle of 

 the convex side of the crescent, or the illuminated limb of the 

 moon ; and we shall still apply the name, even when the moon is no 

 longer a crescent. The sagitta is necessarily aimed directly at the 

 8nn ; and yet, to the eye, it always seems to point above, often very 

 considerably above, the sun. The reason is that we are accustomed 

 in drawinir. and looking at, pictures on flat surfaces, to consider and 

 treat points or forms as they are projected on the '' plane of 

 yision " ; but they aro really presented to us as projected 

 on a spherical surface, whose centre is the eye. The differ- 

 ence is trifling when the angular field of view is small, and 

 from habit we neglect it, and we are led to do this even when 

 the difference is very imponant, and we know it to be so. Take the 

 simplest case of the illusion mentioned. The sun is on the horizon, 

 and the moon is 90° distant, not perceptibly different from half- 

 moon ; we shall suppose her altitude to be 25°. The moon is at the 

 highest point of the great circle of the sphere of vision passing 

 through the sun, the moon, and the point of the horizon opposite to 

 the sun. As the sagitta points directly to the sun, it lies on that 

 great circle ; but as it is at the highest part of that great circle, it 

 is horizontal. Now, when wo jiroduce the horizontal sagitta sun- 

 wards by the eye, we cannot help picturing to ourselves a line which 

 remains always parallel to the horizon, and our mental production 

 of the sagitta, instead of hitting the setting sun, passes 25" above it. 



The illusion is connected with the fact that, in turning to look 

 from the moon to the sun, the observer rotates his head, or perhaps 

 his whole body, round a vertical axis, and not about an axis perpen- 

 dicnlar to the plane containing his eye, the moon, and the sun. By 

 taming himself properly in the latter manner, the obscner can, by 

 the eye, correctly produce the sagitta so as to hit the sun ; it will 

 be better, though not necessary, to screen off the horizon. We have 

 neglected refraction, the effect of vhich is small comparatively, but 

 in the case considered, the difference of refraction goes to diminish 

 the illusion, and not to help it. M. 



[Five or si-t years ago I put tho question raised by " M " before 

 the readers of the English Mechanic. It is connected with a question 

 of some interest to artists, viz., the true rules of perspective for 

 pictures including a very large visual area. — Ed.] 



LATIN QUOTATIONS.— OPTICAL ILLUSION. 



[24] — I welcome with much pleasure the appearance of your new 

 scientific journal, and feel sm-e that it will meet with the support it 

 deserves. It is not cvei-yone who cares to pay sixpence for Nature, 

 and the cheaper ones arc sadly wanting in tho tone and character 

 which I think should pervade everything connected with science. 

 I should like, however, to make one dejirecatory remark. I think 

 that the too frequent use of Latin quotations should be avoided in 

 a paper addressed principally to those who have not had an uni- 

 versity education. Many who take a lively interest in science have 

 not had time to study the dead languages,* and to them (unless 



• [There arc quotations so familiar that they can scarcely be 

 regarded as belonging to a dead language. Such was the one I 

 nsed, Nescit vox miitsa reverti, which means that " what has been 

 said cannot be unsaid." — Ed.] 



accompanied by a translation) these quotations are simply 

 repellent. 



I enclose an instance of an optical illusion just observed. It 

 will bo noticed that Mr. Smith's address appears to decline to the 

 left. — Yours trulv, 



Xoi-. 9, 1881. ' C. J. Watson. 



SMITH, 



Blank Green, 



SMITHBOEOUGH, 



Near BLANKTtlWN. 



OPTICAL ILLUSIONS. 

 [25] — The No. 1 illusion I made an indopendent discovery of 

 many years ago, as also, no doubt, have many more who have been 

 much used to drawing geometric and mechanical patterns. Since 

 the advent of Knowledge, however, I have thought a little more 

 over it. Tho e.tplanation of this figure is, I believe, to be found in 

 the peculiar movements of the muscles of the eyeballs. The prin- 

 cipal movements of the eyeball are vertical and horizontal. Vertical 

 lines are followed b}- means of one pair of nmsclos, which move 

 the eyeball tlirough a vertical plane. Horizontal lines are 

 followed by corresponding muscles. Oblique lines are followed 

 by the eye through the combined action of both those pairs 

 of muscles. It is true that there is also an oblique pair 

 of muscle.?, but from their position, I do not think they assist the 

 eye to follow an oblique line, being more suited to rotate the eye 

 upon the axis of its lenses. Eeferring now to diagram No. 1, the 

 eye follows the line EG by a simple movement, but in following the 

 Ime AB, both pairs of muscles are employed. This movement 

 being more complex, is not so certainly continued when the guide 

 of the line is lost by the interposition of the space EH. The eye 

 is led away, as it were, also by the commencement at the point B 

 of the line BG, which breaks the continuity of the oblique move- 

 ment. It seems to me further complicnted b_v tho movement of 



Fig. a. 



Fig. b. 



two eyes, -A'hich work more easily together in the vertical and 

 horizontal movements than in the oblique. Tliis may be noticed 

 when closing one eye, the illusion being then not so complete. 



In confirmation of my remarks, I send yon two diagrams. If the 

 line EG be broken at B, tho illusion is almost destroyed, tho line 

 having lost its power of attracting the eve to a simpler movement 

 (Fig.o). ■ 



Again, if the same diagram bo set before the oye symnictrically 

 disposed to tho vertical position, so as to present to the eye similarly 

 situated oblique lines, the illusion will vanish, as then tho oblique 

 movements of the eyes are refjuired to follow both the parallel lino 

 and the one which is opposed to them. Tliis, then, brings the diagram 

 to the form of an ordinary Koman X (Fig. h) in which there is no 

 difficulty in following the single lino. If this same diagram bo set 

 before the eye with the parallel lines either vertically or horizontally 

 disposed, the illusion will reappear. Hoping Knowledge will be as 

 successful as it deserves, I remain, yours, &c. 



W. D. RlUIMOND. 



