October 22, 1908] 



NATURE 



6x\ 



divided into seven parts or chapters, which deal re- 

 spectively with the following; subjects : — (i) The enu- 

 meration, description, and illustration of woodwork- 

 ing tools, such as benches, squares, chisels, saws, 

 planes, brace and bits, &c. ; (2) woodworking 

 machinery, including band and circular saws, wood- 

 planers, and wood-working lathes; (3) the classifica- 

 tion, description, and properties of various woods ; 

 (4) fastenings, such as nails, screws, glue, dowels, 

 cleats, &c. ; (5) the finishing of wood surfaces by 

 paints, stains, polishes, and varnishes ; (6) in this part 

 we have a graduated set of examples, with dimen- 

 sioned drawings, of suitable objects to be made at 

 the bench, beginning with simple knife-work and 

 ending with a combination desk and bookcase ; 

 and in (7) instruction is given in wood turning, with 

 examples for practice. There are three appendixes 

 giving some problems in practical geometry, some 

 useful tables and instructions, and a key for the 

 identification of all the principal w'Oods of North 

 America. 



As will be seen from the foregoing, the book is 

 intended to be used under the direction of a skilled 

 workman, who is responsible for giving instruction 

 in the proper handling and manipulation of the tools. 

 Teachers in this country will be well advised in con- 

 sulting this e.xcellent text-book. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



On the Change of Colour in the Eyes of an Attis 

 Spider. 



On Saturday, September 5, I found a small spider with 

 light green, transparent legs and brown body with silver 

 flutings. I bottled it quicldy, and hurried up to my friend 

 Mr. Strickland, and on e.xamining it there under a mag- 

 nifying glass observed a frequent change of colour in 

 its eyes. I took it home, and on examining it for about 

 six hours consecutively found it to have the faculty of 

 changing the colour of its eyes at its own free will. In 

 an instant it changed the honey-coloured eyes into shining 

 black. While it changes the eyes, a bright dot or streak 

 appears and vanishes all at once. 



I am quite sure that the animal actually changed some- 

 thing inside the eyes. The cornea — as one may call it — is 

 circular. The two corneas stand in a vertical plane so 

 that they face the observer lil-ce a pair of gig-lamps, or, 

 still better, as those in front of a railway locomotive. 

 Behind each cornea is a conical sack, in shape much like 

 an ordinary butterfly-net or a jelly bag. Taken together 

 with the cones, the pair of eyes look like a pair of field 

 glasses. The spider was found to wag the conical portion 

 of the eyes every now and then. Fortunately, the head in 

 this species being translucent, the mechanism by which 

 the colour-change is effected can be easily seen by means 

 of a good pocket lens. The spider itself was 6 mm. in 

 length, and its conical eye one millimetre. 



I put the spider in a small, thin, clean test-tube, and 

 stopped the mouth of the tube with a little bit of cotton- 

 wool. Having done this, I took the tube to a powerful 

 table lamp and examined it with a pocket lens in that 

 light against a white background. A thin strip of white 

 paper serves very well as a background. When I first 

 took it near the light the spider seemed to be startled and 

 ran about. It was at this moment that I saw it wagging 

 the conical part of the eye all the more. The spider ran 

 a few paces, then stopped, and began moving the eyes 

 very vigorously. On closer examination I found that the 

 outer and larger end of the cone was a transparent honey 

 colour. The inner tapering portion of the cone was jet 

 black. The light and black halves were divided by a well- 



N'O. 2034, VOL. 78] 



marked ring. The change in the colour of the eye is 

 caused — as will be explained immediately — by the wagging 

 to and fro of the two posterior cones. Reference to the 

 diagram will show that the cones can be in such a position 

 (A, A) that their axes are parallel to one another and in 

 the line of sight of the spectator facing the cornea, or 

 they can converge to a point just halfway between the two 

 eyes in question (B, B), or the axis of one eye may con- 

 verge while that of the other will remain unchanged. It 

 is to be observed that the apices of the cones never diverge. 

 Roughly speaking, the black extends only one-third of 

 ihe whole length of the cone from their tips. Consequently, 



lEFr 



RIGHT 



Fig. I. — (i) Position A, A. Both the eyes of the spider now appear black, 

 because the line of sight plunges into the black portion of the cone. 

 (2) Position B, B. Both the eyes now appe.ir honey-coloured, because 

 both lines of sight cut the honey-coloured parts of the cones. (3) Posi- 

 tion A (right), B (left). The right eye will appear black, the left hoaey- 

 coloured. 



when the spectator faces the eyes, and the axes of the 

 cones are parallel, he sees into the depths of the two cones, 

 and the eyes necessarily appear jet black. When the two 

 tips of the cones converge the line of sight strikes the 

 honey-coloured outer portion of the cones, and then the 

 eyes in consequence appear honey-coloured. Lastly, the 

 sJDider has the power to cause the tip of only one cone 

 to converge inward, and then only that eye appears honey- 

 coloured, while the other one remains black. It has been 

 stated above that when the spider changes the colour of 

 the eye a bright line or dot traverses the cornea. This is 

 due to the ring formed where the black and honey-coloured 

 portions of the cones unite traversing the cornea as the 

 colour of the eye changes from light to dark, and vice 



Fig. 2. 



versd.. It must be well borne in mind that in all these 

 cases the cornea of the eye remains perfectly unchanged and 

 immobile, the change of colour being wholly and entirely 

 due to the movement of the cones behind it. 



When the line of sight from the observer's eye to the 

 cornea is at right angles to the latter the eyes invariably 

 appear honey-coloured. The reason is obvious, namely, that 

 the line of sight strikes only the honey-coloured portion of 

 the conical sack behind the eyes. Hence it follows that 

 the axis of the cones must be either above or below the 

 line of sight. But as a m.atter of fact it is above it. 

 The proof of this is that if you look at the eyes a little 



