HARDWICKE'S SCIENCE-GOSSIP. 



135 



rial, it is likely that these glands and tubes pro- 

 duce it. 



But, on the other hand, in the Epeirse, on each of 

 the first spinnerets are three spinning-tubes of a 

 still more peculiar character than the large tubes 

 on the third spinnerets. These are very well shown 

 at fig. 89, b, and there is a larger figure of one at 

 fig. 88, c. Not only do they differ in shape and size 

 from the rest of the tubes on the spinneret (com- 

 pare c and d, fig. 88), but they are placed on a 

 thickened plate of chitine,and the glands belonging 

 to them are of very large size. In a large Epeira 

 they measure & or ■$* of an inch in length, while a 

 common silk-gland (e, fig. 89) is about yh; of an 

 inch. Like the similar glands of the third spin- 

 neret, they are not represented, because they are too 

 large; but their three ducts are drawn, and it will 

 be seen that they have a covering of most curious 

 globular cells (perhaps themselves glands), which 

 are so slightly attached to them as to be easily 

 rubbed off during manipulation (see the duct 

 marked x +). At present I have been unable to 

 detect anything analogous to these glands in the 

 spinnerets of any other genus, except in a foreign 

 spider, a Nephila, which seems very nearly allied to 

 the Epeirse. Now I believe that the Epeiridse are 

 the only family of (British) spiders which spin 

 viscid webs ; wherefore I think it not unreasonable 

 to conclude that it is the office of these glands and 

 spinning-tubes to produce the viscid gum. 



This conclusion, if correct, goes against my 

 notion that the viscid threads are formed by the 

 third spinnerets ; but one thing I may mention — 

 I am sure non-viscid threads are produced by the 

 first spinnerets, because I have seen them in the 

 act of being spun. 



In fig. 89 are shown a few of the glands belonging 

 to the common spinning-tubes, a (see also d, fig. 88), 

 which secrete the silk. These are the glands cor- 

 responding to that one shown at fig. 121, p. 181, in 

 Science-Gossip for 187-i, as belonging to a Tege- 

 naria. It will be seen that, although different in 

 hape and somewhat smaller, their construction is 

 essentially the same. They vary a good deal both 

 in form and size, in different individuals. At b, 

 fig. 89, on the second spinnerets, are shown two 

 extra large spinning-tubes, which may have some- 

 thing to do with the viscid globules ; and at d, on 

 the same figure, is shown what I believe to be the 

 gland belonging to them. I am uncertain whether 

 it has a blind end, or whether it expands into 

 a sac. 



At C, fig. 90, are some of the ordinary glands of 

 the third spinneret. My description of the corre- 

 sponding glands in a Tegenaria was, as I have since 

 found, very incorrect ; so I will just describe these. 

 A gland consists of, first, an outer membrane, g, on 

 the inner surface of which is a layer of columnar 

 epithelial cells,/. These absorb, by endosmosis, the 



constituent parts of the silk secretion from the 

 fluids in the body, and discharge it as perfect liquid 

 silk into the caeca or gland-tube e. Erom this it is 

 conducted by the duct (h) to the spinning-tube (a). 

 This description of the method of secreting the 

 silk applies equally to the other glands. 



H. M. J. Underhill. 

 (To be continued?) 



MICROSCOPY. 



Prismatic Mounting Slips.— It is a fact well 

 known to microscopists that structural detail is 

 more easily detected when viewed in an oblique 

 direction. Oblique vision must not however be 

 confounded with oblique light. In the former case 

 the object forms an angle with the eye or object- 

 glass ; in the latter the object lies parallel, and 

 merely illuminated by an oblique pencil of light. 

 The closeness of the front lens in objectives of high 

 magnifying power prevents the object being viewed 

 in an oblique position. To overcome this difficulty, 

 Mr. F. H. Wenham proposes the following plan :— 



(V 



nz 



Fig. 91. Prismatic slips. 



A slip of glass -^ wide has one of the edges 

 polished off to an angle of 35°, if for dry objects 

 (for balsam mounting an angle of 45° is preferable) . 

 The above diagram represents a slide, with pris- 

 matic slips a and b attached. The objects to be 

 mounted, such as diatoms or insect-scales, are placed 

 as near the knife-edge of the prism («) as possible ; 

 those situated nearest the edge may be viewed by 

 the highest powers. The slip thus prepared may 

 then be fixed with a drop of balsam on an ordinary 

 slide. Another slip (A) has its incline pressed 

 against it so as to lay the object flat against the 

 two inclines. The lower prism is necessary, for 

 without it a deal of offensive colour enters the 

 object-glass from the decomposition of the trans- 

 mitted light. This is recomposed or neutralized 

 by the under prism, which also greatly increases the 

 obliquity of the ray by refracting this to the same 

 an<?le as that of vision from the deflection of the 

 axial ray of the object-glass. A more fully detailed 

 account of the action of these prismatic slips and 

 the method of miking them will be found in the 

 April part (No. 76) of the Monthly Microscopical 

 Journal. 



Microscopic Query— I shall be glad^to hear 

 from any correspondents who possess a " Beck's 

 Popular Microscope" what they have been able to do 

 with their quarter-inch in the way of resolving best 



