266 



KNOWLEDGE ♦ 



[Sept. 26, 1884. 



action of the brain-cells is motion, i.e., changed motion, i.e., 

 thought. In this eense only I understand Biichner to say, 

 '* thought is motion." 



As to the power which determines the nature of the molecuhir 

 changes, I take it to be the simple force of circumstances in con- 

 junction with the nature, power, or capacity of our brain-cells. 

 Jnst as different light, such as star, moon, or sunlight, produces 

 different colf.nrs in passing through a prism (?) or through various 

 sorts of gliiss or in different positions, so will, say, different kinds 

 of stimuli produce different thought in different brains. For in- 

 stance, a lecture on geology or astronomy would produce different 

 kinds of thought than one on theology, Ac. F. W. H. 



THOU DUST! 



[1415]— We appear to be threatened with a recrudescence of the 

 sunset phenomena of last year. May I venture to suggest to the 

 members of the Krakatoa Committee, through the medium of 

 Knowledge, that even if they abandon the volcanic theory, they 

 need by no means abandon the hope that the cause is a purely 

 terrestrial one. 



During the enmmer people go from home ; in their absence the 

 carpets are beaten, the fine dust rising into the atmosphere pro- 

 duces the observed effects ; the colours are intensified by the 

 fragments of dyed wool. Voilii tout '. 



One slight apparent difiBcnlty remains, viz., an explanation of the 

 fact that these effects have only been noticed recently ; but a com- 

 mittee whose ability easily coped with the fact that the glow from 

 the dust was seen long before the eruption will doubtless readily 

 dispose of such a trivial and captious bit of criticism. F.G.S. 



[It seems to me that there is as much— or as little — to carp at 

 in " F.G.S. 's magnificent theory as there is in that held by the K.S. 

 Committee. — Ed.] 



FALSE PERSPECTIVE. 



[The following letter was inserted in our last issue, but the 

 diagram was inadvertently omitted. We therefore repeat the 

 letter in its complete form. — Ed.] 



[1399] — If I do not trespass too much on your space and patience, 

 I shall be glad if you will kindly insert the following in answer to 

 your correspondent, "J. H. D." (No. 1386.) 



..-^.. 



if. 



If an object at A appears smaller than a similar one of equal size 

 at B, lecau.ie it is further off, so likewise, and for the same reason, 

 will it appear smaller than B when it is moved to b. 



The painter must depict things as they appear to the eye ; it is 

 for the sculptor to represent them as they exist in nature. 



K. Jones. 



[1416] — I fear my letter (1399) without its accompanying 

 diagram, which has been, I presume, accidentally omitted, will not 

 be very readily understood. I do not purpose troubling you with 

 any further remarks on FaUe Perspectire, but shall be obliged if 

 you will kindly print this, my final answer to " Eye Witness," 

 " Old Draughtsman," and others who have taken part in the 

 discussion. 



" Eye Witness " is right — the horizontal and vertical lines will, 

 doubtless, have vanishing points, which if not quite so far off as 

 the nearest fixed stars, are, nevertheless, sufficiently remote to 

 render their inclination practically inappreciable. If " Eye Wit- 

 ness " will refer to my former letters he will perceive that the 

 drift of them has been to establish the fact, that when an observer 

 is so placed that he can see a portion of (mo sides of a rectangular 

 solid, the top and bottom edges of these sides will be seen to trend 

 away in the manner I have described. 



"Musafir" objects that my line of posts, if produced both 

 ways, would converge to the right and left. " Old Draughtsman " 

 (in letter 1288) furnished a sketch of a portico whose columns 

 from the centre one right and left diminished in height very rapidly 

 indeed. The effect was certainly peculiar, but theoretically true to 

 Nature, although the diagram in question was manifestly intended 



to afford ia sort of reductio ad al.iurdum proof of the fallacy of 

 my own views. If " Old Draughtsman's " grotesque presentmer/i 

 did in any degree answer its inventor's purpose, it was, I fancy, 

 because the details of the drawing were so monstrously inartistic 

 and unscientific in their exaggei'ation. R. JON'ES. 



Sept. 19, 1884. 



[1417] — I have read attentively all the letters on the above sub- 

 ject which have appeared in your columns, and have several times 

 been on the point of writing one myself, but have hitherto refrained . 

 But, after the letter of " An Old Draughtsman " in your last 

 number (150, p. 224), I would like, with your permission, to say 

 something about it. The theory of " A. O. D." that the pictun^ 

 plane can assume other positions than at right angles to the Unc 

 of vision (or direction in which the spectator looks) is surely utter 

 nonsense. Taking Q Q (see his first figure) as the position of tb'- 

 picture plane, he makes out that the post / must be drawn about 

 twice as high in the picture as the post a. (A a is retsiined in all 

 his examples as the direction of vision.) Now, let reader? 

 produce the line fa to the right of a, and place a post, 

 say g, as far to the right of a as / is to the left. Join A^. 

 produce Q Q to cut A3 inn. The distance of n from A 9 will 

 give the distance from the post a or / in his second figure, at 

 which it has to be drawn to show its apparent size in the picture. 

 Draw it between z 2 and eye of spectator, A. It will be abont half 

 as long as z J, or one-fourth of yy. Call it n n. It seems, then, 

 that the posts / and g, of equal height, placed at equal distances 

 from the spectator, and at equal distances to the left and right nf 

 A a, the direction of vision, may be represented in a picture by lint^ 

 in the proportion, say, of 4 to 1. Such a picture would be much 

 after the style of the distorted image which one sees of one's faci' 

 on looking into a silver spoon or a teapot, at the same time turning 

 the head round a little. Surely " A. O. D." is making fun. It is 

 quite true that Q Q may be used as the position of the pictnn- 

 plane, but then the spectator must turn somewhat round, so as t" 

 look in a direction at right angles to Q Q, which, of cotirse, alters tli> 

 whole case. Taking the problem as it stands, the only possibh- 

 position of the picture plane is parallel to a f, that is, at right angles 

 to the direction of vision A a. How would " A. 0. D." represeni 

 the post /in his picture if his picture plane Q Q were inclined at a 

 somewhat greater angle than represented, so as to make it pass 

 through the line of posts, or behind f? 



With regard to R. Jones's initial difficulty abont the drawing of .•> 

 cube, of which two sides are seen, one, parallel to the picture piano, 

 being represented by a square, it may be remarked that elementary 

 text-books on perspective show how to place, say a cube, in every 

 part of a picture, and in all positions. Bat each of these is only :> 

 small object in a large picture, and seldom in the centre. In order 

 to get the representation of a cube which R. Jones expected, tlu* 

 cube must be placed directly opposite the spectator, with botl> 

 visible sides forming angles with the picture plane. It will thru 

 occupy the centre of the picture, and both visible sides will vanisli. 

 The same will happen with R. Jones's original cube (which is t" 

 the left of the direction of vision) if the spectator turns round and 

 looks straight at it. Of course, this changes the direction of vision, 

 at right angles to which a new picture plane must be drawn. W' 

 will then have, as before, both sides vanishing. 



A curious fact may be mentioned about the perspective repre- 

 sentation of a sphere not in the centre of the picture. The rays <vf 

 light from all parts of a sphere to the observer's eye form a coni-. 

 If the sphere is some distance to the right or left, this cone will be 

 cut obliquely by the picture plane, the section being an ellipse. -\ 

 sphere so placed will therefore be correctly represented in tln^ 

 picture by an ellipse, and not by a circle, as one would at first 

 suppose. Stakch. 



AERIAL LOCOMOTION. 



[1418] — The interest with which I read your recent articles on 

 aerial flight was not unmixed with curiosity, and ultimately, I may 

 confess, with some disappointment at the absence of all referent-*- 

 to one possible solution which has of late years obtruded itself on 

 my speculations on this subject. The method to which I allnde is 

 that of ascent into, and continued support in. the air by the help of 

 a screw-shaped fan . I do not know whether it has ever been tried, 

 nor, indeed, can I call to mind any distinct proposal of this kind ; 

 yet it appears to have elements of success which might well tempt 

 mechanical invention, and must surely have done so. 



If a screw can be made large enough, light enough, and strong 

 enough to raise the weight of a man, in addition to the machinery 

 and reservoir of power requisite to turn it at the proper speed, the 

 problem of aerial locomotion will surely be solved ; for it will 

 obviously be only necess:iry to incline the a.cis of the screw in order 

 to obtain lateral motion. 



