Nov. 28, 188-t.] 



• KNOWLEDGE 



441 



Fig. 31. 



illusion referable to some extent to this property of reten- 

 tion which the eye potsesses, and to instruct the student 

 how to construct the nece.^sary apparatus for exhibiting 

 it. The toy — if it be a toy, and not rather a refined philo- 

 sophical instrument — was brought out a good many years 

 ago, under the name of tlie Anorthoscope, from two Greek 

 words signifying to set the vision right. That it is very 

 approi)riately named the reader will immediately see. It 

 seems hard to understand why it has so utterly fallen into 

 oblivion. As our object here is rather to provide the 

 student with striking and amusing illustrative experiments 

 than to enter into expositions of recondite optical laws, 

 we will proceed to our description at once. If, then, a 

 figure, distorted according to principles immediately to be 

 explained, is delineated on a disc, and tbis disc turned 

 round, while a black disc with four radial slots is caused 

 to rotate four times as fast, and in an opposite direction 

 in front of it, the spectator will see five stationary images 

 of this distorted figure, but restored to its natural pro- 

 portions ! 



Fig. 32. 



FiEf. 34. 



First, then, to draw the figure, which we will suppose, 

 for the sake of illustration, to be a butterfly. We begin by 

 striking two circles (Figs. 31 and 32), dividing them by 

 radial liues at any convenient distance. In Fig. 31, such 

 division is made at every 5°. Only half the circle, or 180°, 

 is so divided in our figure ; but Fig. 32 was similarly 

 dividfd right round the circle for a reason which will be 

 immediately obvious. We also strike a series of circles 



interior to our larger one to fix certain points in our 

 drawing. Very well ; incur first divided circle (Fig. 31) 

 we now proceed to draw the best butterfly we can, and, 

 having finished it, note with the greatest care where the 

 diSerent points of the figure fall. In copying it, we must 

 be careful to preserve the radial length or central distance 

 of each part with the utmost accuracy, hut v:e must expand 

 it laterally to Jive limes its orijinal angular dimensions. 

 For example, the tips of the butterfly's antennae are each in 

 circle 7, and the lines marking .">° on each side of the 

 median one. Hence, we must put the same tips in Fig. 32, 

 still in circle 7, but on the lines marking 25° {i.e., 5 x S^). 

 The upper corners of the wings fall on circle 6, and on the 

 lines mai-kiug 25°. The corresponding points in Fig. 32 

 are, of course, as before, on circle 6, but on the 125° (or 

 5 X 25°) lines ; and so with the markings and other leading 

 features. 



It now only remains to join the points thus obtained, 

 rub out our division lines, and we have Fig. 32. Fig. 33 

 shows the disc of black cardboard with four radial slots, 

 which is to rotate before the diagram which we have just 

 produced, and Fig. 34 represents in section the apparatus 

 by the aid of which the necessary motion is given to them 

 both. Here S is a stout, square, wooden stem on a stind, 

 through which pass two axes — -f, a, a fixed axis, and m, a, an 

 axis moving with the wheel upon it. On the fixed axis 

 rotate two pulleys (or grooved wheels) p p'. obviously inde- 

 pendently of each other. A third pulley p" is immovably 

 attached to the axis in, a, so as to turn with it. These 

 three pulleys are all of precisely the same diameter. The 

 wheel u; also fixed to the axis ma, is four times the 

 diameter of ^^ p\p"- Hence it will be seen that if endless 

 cords ec, e, c, go round p',p" and w,p respectively, p will 

 rotate four times as fast as p. It will be noted that the 

 cord e' c is crossed iu order that p may turn in an opposite 

 direction to />' also. Finally we attach our black disc with 

 the radial slots, 33, to p and our distorted figure (32) to p', 

 light the latter well, turn the handle /;, and look through 

 the slots. The spectator who sees the five stationary butter- 

 flies for the first time, will, we venture to think, hardly do so 

 without an exclamation of astonishment Of course, any 

 figure whatever may be distorted on the same principle. 



It is stated that the entire number of the cases of cholera, and 

 of the deaths from it, at Naples has now been computed. In the 

 city of Naples there were 12,402 cases and 6,629 deaths. In the 

 whole of the province of Naples there were 14,037 cases, and 

 7,576 deaths. 



