January. 1911. 



KXOWLiiDGE. 



19 



though the stars under observation (or one of them) 

 are too faint to permit of continuous vision. Bright 

 wire illumination on a dark field is verv useful in 

 such cases, but this may be beyond the reach of 

 many amateurs. 



A second pair of distance " wires " of thicker web 

 will often he found verv useful ^\■hen faint stars have 

 to be measured, and the illumination of the field has 

 to be kept down so low as to render the ordinary 

 webs invisible. For these thicker "wires" the guy 

 lines supporting a web may be used. The}- are of 

 about the thickness of a fibre of silk and consist of 

 many ordinarv lines of web made b\- the spider into 

 what ma\- be called a cable. 



It will occasionallv happen that the " w ires " 

 (spider webs) of a micrometer break, as the result, it 

 may be, of lack of care in handling the instrument, 

 or even o\\ ing to the weather. This may be a source 

 of inconvenience of a serious character to an observer 

 if he is living at a place remote from an optician. 

 A thick metal wire is renewable without much 

 difficulty by anvbody possessed of a moderate amount 

 of mechanical skill, but to replace a genuine spider 

 line may be a matter of some difficult\', first of all to 

 obtain such a thing, and then to fix it. 



In order to obtain a supply of spider line take a 

 piece of moderately stout iron or copper wire about 

 9 inches in length and of such thickness (sa\' 22 

 Birmingham wire-gauge) as to be fairl\- rigid when so 

 bent into the form of a 2-pronged fork that the 

 ends may be about three inches apart. Ne.xt, hunt up 

 a spider and persuade him to attach his web to the 

 wire fork near one end and then cause him to drop 

 a few inches and catch the web on the other prong 

 of the fork : then b\- turning the fork round one can 

 wind up as man\- turns of the web as one cares to 

 ha\e, taking care to keep each new turn separate 

 from, that is, not in contact with, the previous one. 

 If this part of the business has been satisfactorily 

 accomplished a stock of spider line will have been 

 laid up for use. 



The next thing is to render the lines secure on the 

 fork before selecting and removing one preparatory- to 



transferring it to the micrometer. This is done bv 

 t(-)uching both ends of each \;urn of the coil where it 

 jiasses round the fork with a droj of shellac varnish 

 or other sticky stuff. When this has set and is dry, 

 the removal of any one web mL y be accomplished 

 without affecting the others. 



\Mien it is desired to take a web < .vit of stock in 

 order to use it, make a second stout wire lork, rather 

 less in breadth between the prongs than the first 

 one. Put some sticky stuff on the tip of each prong 

 and lift off a length of line from the storage fork. 

 Remove the cover of the micrometer : clean off any 

 old ends of broken web : touch with \arnish each 

 spot where the ends of the new- web are wanted to 

 go, and drop the new web into its proper place ; let 

 the varnish dr\- ; and when the web is found to be in 

 the right place and secure, cut off any overplus with 

 a small pair of scissors. This final action in the 

 work of renewal is more easily described than carried 

 out, but with a little patience and the spoiling of a few 

 webs, a final good result mav reasonably be expected. 



If \ou cannot secure the cooperation of a living 

 spider to provide a new web, and have none in stock, 

 search must be made for an old web in situ, and a 

 suitable portion picked out and ren-ioved with a 

 forked wire (such as already described) and baited, 

 as it were, with some stickv stuff. A web thus 

 casuallv obtained n-ia\- probably be dusty, and may 

 perhajis consist of more than one single strand, in 

 which case certain obvious precautions must be 

 taken and things done. 



It will seldom happen that marks cannot l>e 

 detected on the sliding fork of the n-iicronieter to 

 indicate where old webs have been attached, and 

 therefore, where new ones must be applied so that 

 they may be parallel to one another. Use a screw- 

 driver which fits properh- the small screws of the 

 micrometer when it has to be taken apart. A sharp 

 bradawl is a good substitute. Nothing looks worse 

 than the head of a screw- damaged b\- a tool unfit for 

 work. While the screws are out, put them in a little 

 box for safe keeping, for small micrometer (and 

 other) screws have great straying power. 



SPECULATIONS. 



In the volume of which I have made so much use, 

 Innes has entered upon some reflections and specula- 

 tions which, as the_\- are not of the wild and 

 irresponsible character so common now -a-da\-s. I 

 gladly quote b\- way of conclusion to this prelhninar\- 

 dissertation because of their present interest and 

 prospective usefulness. He says : — 



" The study of Double Stars will always be 

 interesting from the Newtonian point of view, and 

 in the case of the brighter pairs, the spectroscopic 

 determination of motion in the line of sight will lead 

 to a knowledge of the true position in space as well 

 as of the absolute dimensions of their orbits, and 

 hence their paralla.xes. The irregularities of their 

 orbital motions (already ascertained in a few- cases. 

 which will multiply as observations grow more 



precise) indicate the presence of disturbing bodies. 

 For these reasons even the Binaries w ith the best- 

 determined orbits deser\-e careful and regular 

 observation. From the astro-physical point of \-iew, 

 it is hoped that much max- be learned from their 

 stud\-. A contracting and rotating mass of gas may 

 e\-entuallv reach an epoch when it will become 

 unstable, and separate into two or more portions 

 forming a Binar\- system. It is evident that this 

 critical period will depend on the constitution of the 

 body, its speed of rotation, mass and temperature. 

 S\-stematic observations, micrometric and spectro- 

 scopic, should in time enlighten us on these, 

 at present, obscure subjects. When we are able 

 to arrange the elements of the orbits of say one 

 thousand Binary pairs in tabular form, with their 



