July 18, 1884.] 



♦ KNOWLEDGE • 



61 



"tiny ghost," and "corpse-like thing" referred to, thongh on this 

 point we can scarcely speak definite!}* in the absence of a more 

 accurate description. The pupa state usually lasts about six weeks, 

 but its duration would be largely dependent upon temperature. 

 The beetle, on emergence from the pupa, is soft and of a reddish 

 colour, but, after a while, hardens and darkens. 



The query as to rariation in size and growth of perfect insects 

 will be found answered in Knowledge, Xo. 138, p. 450. Sex and 

 differences in quality and quantity of food are the principal factors 

 in producing differences in size. Amongst beetles, this variation is 

 greatest in those that feed on solid wood. E. A. BcTLER. 



7, Turle-road, Tollington-park, N. 



TRUE AND FALSE PERSPECTIVE. 



[1341] — A perspective drawing of an object may be defined as 

 any section of a sheaf of rays of light proceeding from that object 

 to a given point, at which the eye may be placed. 



If the section be correctly made, the perspective drawing is 

 absolutely true in all cases, and on the application of the eye to 

 the apex {i.e., the given point) of the sheaf of rays the repre- 

 sentation on the section has precisely the appearance of the object; 

 or if the representation be made opaque it will exactly hide the 

 object from the eye. If we decide on making a vertical plane sec- 

 tion of the sheaf of rays parallel to the face of the object, that 

 flecision becomes the governing element in the problem. 



Ground Plan. 



In the accompanying diagram 1, 2, 3 are ground plans of cubes. 

 The spaces marked Al, A2 ; A3, A4; A5and A6, are eqaal to each 

 other ; the sides A, B are also equal. To the point S lines are drawn 

 from all the angles of the cubes, which would be seen on placing 

 the eye at S. These lines may be called the leading lines of light 

 from the objects to that point. The section line C, D is parallel to 

 the object, as decided. 



The spaces on the section line mEirked al, a2 ; aS, ai ; and a5, 

 a6 are equal to each other in the section as they are in the objects. 

 It is evident that on placing the eye at S the space at al, a2 will 

 exactly cover the space Al, A2. All the other spaces of the sec- 

 tion will exactly cover all the corresponding spaces of the objects 

 in like manner, and if we make them opaque they will hide the 

 spaces of the ground-plan and stand for them. 



What has been demonstrated of a ground plan is equally tme 

 of a vertical section. JoHX Bacox. 



STRAKGE SKY EFFECTS IN NEW ZEALAND. 



[1342] — I have just received a letter from a young niece in New 

 Zealand, dated May 20. She writes from near Christehurch, 

 Canterbury : — " We have been having the strangest weather lately, 

 and such curious skies for some months past. The other morning, 

 about 11 a.m., I looked out towards the east, and the sky was all 

 golden and crimson as if it were stinset, as if the sun had made a 

 mistake and was going to bed at the wrong time and in the wrong 

 place. At night, we have the most lovely, rosy lights in the sky : 

 bitt, beautiful as the skies are, and much as one admires them, they 

 seem to give one a sort of uncanny feeling as if something were 

 going to happen." Cosmopolitan. 



LETTERS RECEIVED AND SHORT ANSWERS. 



Seneca. If yon are going to try to teach yourself French, pro- 

 bably the "French" in Prendergast's "Mastery Series" is as good 

 a book as yon can get. — Foreigner disagrees with Delille's classi- 

 fication of " mon," "ton," "son," with " le mien," " le tien," 4c. 

 — W. Thanks; marked for insertion. — Edwin W. Morris and A. 

 Mackay. I regret to say that I have been compelled to relinquish 

 lecturing entirely — at all events for a considerable time to come. — 

 M. B. Obviously a misprint, the figures being transposed. — H. G. 

 has found some earwigs in strawberries ! Well, they are not 

 pleasant to look upon ; but, doubtless, in the words of the poem, 

 they are " uonrishing, very." W. C. B. demurs to the idea that a 

 polluted river is necessarily unwholesome, as over and on the banks 

 of one — "a mass of festering filth, chocolate in colour, molasses 

 in consistency, and of stench simply indescribable" — many of 

 his workpeople live hale and hearty, as did he and a large family 

 for fifteen years. Moreover, twenty cows, always well and thriving, 

 drink this filth in preference to pure spring water. Just so ; de 

 gustilus nan est disputandum ! — F. W. D. Mitchell. Your 

 pamphlet was reviewed on p. 36. — J. P. Bourne sends me 

 a " Handy Assurance Guide," consisting of a tabular mass 

 of statistics having reference to ninety-five of the principal in- 

 surance offices in the kingdom. Intending assurers can gather the 

 position and prospects of any one of them from it at a glance. — 

 E. D. Wabeing saw his own ghost in bright sunshine in 1882, and 

 wants to know whether the comet of that year had a disturbing 

 effect upon light ? Unless comets make people light-headed, I 

 should say, no. He further opines that the brain is affected by 

 light, the heart by electricity, and the lungs by oxygen, and that 

 the difference in the shape of light (!) would make the distinction 

 between man and beast, &c. " I have written," says my corre- 

 spondent, " all this very diffidently, feeling not quite sure whether 

 you may not think it great nonsense." Which — in the most sacred 

 confidence — I don't mind admitting to him that I do. — AKXHrE A. 

 West. Having adopted my suggestion (on p. 16) as to drawing 

 the outline of a cube placed behind, and with one face parallel to, 

 a sheet of glass, finds that, under the prescribed conditions, " the 

 top and bottom margins of the face are parallel." His two remain- 

 ing figures I fail to understand, as one represents a sketch made 

 "on glass facing the centre of the cube" (what is the visible 

 centre of a cube ?), and the other " on glass placed facing away 

 from the cube." This is delightfully vague. — W. A. Cooper. 

 Yon may turn the telescope you wish to test upon the 

 following objects : — Z Herculis, X Ophiuchi, r Ophiuchi, jr AquilEe, 

 4/ Cygni, 49 Cygni (2 ' south-west of f), S Cygni, in twilight 

 only. The ring Nebula between )3 and y Librte, and see 

 if you can see the small star close to and to the east of it. The com- 

 panion to Vega, f' and i- Lyme, in connection with which try bow 

 many stars you can see between the two principal pairs. If the 

 instrument resolves all these tests satisfactorily, buy it. — H. A. 

 Bi-LLEY. When "F.R.A.S." speaks of a planet as "visible," he 

 means visible for the purpose of the observer with the telescope. 

 Perhaps you wiU kindly look at Mars (subtending, as his diameter 

 now does, an angle of less than 6"), and tell us how much detail 

 is visible on his surface, and in what material respect— saving in 

 the exhibition of a rather larger disc — his aspect differs from that 

 of Antares. Mercury is a totally different object, because he is 

 essentially a daylight star, and, under your supposititious con- 

 ditions, would exhibit phases. — H. G. S. It is quite news 

 to me that the erratic genius to whom you refer was an 

 excellent mathematician. But even assuming this to be true, with 

 my present tremendous amount of preoccupation, I certainly 

 cannot spare the time to expose the fallacy of a mere assertion 

 of his, so transparently erroneous, and, moreover, unsupported 

 by the slightest attempt at demonstration. — M. E. Do you 

 seriously expect me to wearily puzzle out the numerical value 

 of 55 figures in succession, with the thermometer standing 

 at 8-1-4 Fahr. in the shade? — Alfred Edward HtTNT. Everybody 

 upon the earth, ex necessitate, partakes of the earth's motion, 

 which affects it utterly irrespective of any other motion that may 

 be impressed upon or imparted to it. The next time yon are in a 

 railway train going 50 miles an hour, drop any object from a 

 point immediately beneath the lamp-glass, and you will find that 

 it will fall vertically beneath that glass on the floor of the 

 carriage, although the carriage has meanwhile moved perceptibly 

 forward. Read Tomhnson's " Rudimentary Mechanics " in Weale's 

 Series. — J. A. E. Have I not said, over and over again, that I 

 must rigidly exclude theology from these columns ? If I admit 

 such a letter as yours, ancf (say) Dr. Aveling or Mr. Bradlaugh 

 were to reply to it, upon what principle of justice could I refuse to 

 insert their' answers ? — E. Jones, having made the experiment 

 suggested on page 16, and finding the result in entire accordance 

 witii the views he previously advanced, offers to forward a proof of 



