KNOWLEDGE. 



TEMPLE O B S E K \' A T ( .) K \' . 



Nan. 



Mas,'". 



Struve's number 22!8 

 h „ 7137 



January, 1911. 



ISSO Appr.ix. Decl. 63° 7J' 



I A. (r5 Cor. for 1900 63' 44' 



jB. 7-7 



(c. 



Gledhill has made the folldwinsj; suggestions as to 

 Double Star observing which he says embody the 

 recorded experience of Strin'e, Sir J. Herschel, 

 Dawes, Dcmbuwski, and Secchi : — 



(II At the outset it must be remarked that the Observatory 

 (doors, windows, slit, and ventilators) sliould be throw n 

 open at least an hour before observation begins, in ordi-r 

 to reduce the temperature of the room to that ot tiie 

 e.xternal air. 



(2) If the definition be bad and the motion great, it is use- 

 less to attempt the measurement of Double Stars. In 

 short, if a power of at least 300 cannot be used, the 

 results cannot generally be of any value. 



(31 Very bright stars should be measured in daxlighl nr 



twilight. 

 (41 The observations should be made near the meridian 



if possible. 



(5) The observer should be in an easy position — the prism 

 effectually secures this ; and the driving clock ought to 

 go smoothly. 



(6) The bright-held should be used almost exclusively — red 

 and blue colours are most in use. 



(7) Use the highest powers possible, and always the same 

 powers. 



(8) A moderate number of measures of an object on each of 

 two nights is better than a large number on one night. 



(9) Use printed forms. 



(10) Enter date, hour, weather, and distance from meridian, 

 before observation begins. 



(11) Notes on definition, general impression as to the %alue 

 of each measure or set, etc., cannot well be too copious. 



(12) In all doubtful cases make a sketch. ,nid .idil full 

 description. ■■ 



.\. few more miscellaneous hints aiiplicablc to 

 telescopic work on Double Stars will now be given. 



Use a dew-cap of a length equal to two or e\-en 

 three diameters of the object-glass; and it is well t<i 

 have the dust-cap tit the 

 saves moving the 

 begun. 



Remove and carefullv 



end of the dew-cap as this 

 itter ever\' time that work is 



cap the micrometer everv 



night to kc'p dust and spiders from getting on to 

 the w ires. 



See that the Right .\scensi(.)n antl Declination of the 

 stars which you are going to measure are for an 

 epoch sufficiently recent that you can make sure of 

 finding the star \-ou want, or you ma\- find yourself 

 measuring the vvrong star. The epoch of 1900 is 

 sufficiently recent, and the principal star of every 

 pair given in these articles is sufficiently bright that 

 there should be no difficulty in dropjiing upon an\- 

 star wanted : but in all cases of doubt the places 

 ma\- be brought up for Precession for any epoch 

 later than 1900 h\ means of the Table of Precessions. 



Examine by da\light the telescope and accessories 

 and see that everything is ready for use in its place 

 and in working order; or. come night and the right 

 star brought into the field, and you may find some 

 of the moving parts of the telescope or micrometer 

 out of order and not be able to rectify matters by 

 lamplight. 



For reading a micrometer small electric lamps with 

 a pinch contact are the best for use ; but whatever 

 lamp is employed, take care that the light is to such 

 an extent shaded as not to endanger the sensitiveness 

 of the eyes. 



Take as many readings as you can remember 

 before writing down any of them, but be sure you do 

 remember them. It is a great advantage to have 

 an assistant to write ddwn the figures whilst the 

 observer is callmg them out. In this way the 

 observer is able to keep his eyes unimpaired liy too 

 much contact with light. Small electric lam[)s of 

 three candle power w ith sw itches fixed to the verniers 

 of the setting circles are valuable accessories. 



Have a movable slide with red, yellow and blue 

 glasses fitted in front of the lamp which illuminates 

 the field, and also a stop+ to control the amount of 

 the incoming light. In this way the illumination of 

 the field can be readily adjusted. It is always well 

 to use sufficient light to ilhnninate the wires even 



' Handbook of Double Stars, page 83. 

 + A -slide of smoky glass shaded off by gradations of tint answers well for this purpose. 



