SCIEXCE-GOSSIP. 



cuai'TIlKs 1'(jr vouxg astronomers. 



Uy Frank C. Duxnett. 



USE OF THE TELESCOPE. 



(Continued from I'oL V.p-375-) 

 One of the first things to be ascertained on pur- 

 chasing a telescope, is the exact magnifying power of 

 each of its eyepieces. To do this accurately, focus 

 the instrument upon .some celestial object, then 

 direct it, during daytime, at the bright sky, as low 

 down as is practicable. If a lens of, say, 2in. focus be 

 applied to the eye, and the light coming from the eye- 

 piece examined, a little image of the object-glass 

 will be found, when the eye and lens are drawn back 

 a certain distance from the eyepiece. Let this 

 tiny image be carefully measured. Personally, I 

 usually eflect this with the thin glass ruled to riV^ths 

 of an' inch, which I use as an eyepiece micrometer 

 with a microscope. We will suppose that the tiny 

 disc of light has a diameter of r^th of an inch. If the 

 object-glass has a clear aperture of 2jin. the magnify- 

 ing power with that eyepiece would be 36. If the 

 aperture of the object-gl.ass is 3in. , then the power 

 would be 48. 



Scuth. 



length. This should be dead blacked inside, either 

 with paint, or with black velvet glued in. It should 

 fit on firmly, in place of the cap over the object glass. 

 This lessens the chance of "dewing"; or condensation 

 of atmospheric moisture on the oljject glass. 



Be careful lo have the instrument well focussed. 

 Difi'erent objects need re-adjustment according to 

 their brightness, and after close work for a little time, 

 the eye needs a slight alteration, e\en when looking 

 at the same object. For examining planetary detail 

 a still night with a slight frosty fog is often the best. 

 The most brilliant nights are not always the best for fine 

 definition, but are available for looking at star clusters 

 and nebulae. In looking for nebulae, star clusters, or 

 comets, the lowest powers with the largest field of view 

 are required, some only being visible with such powers. 

 The Kellner eyepiece is specially constructed for this 

 work. Some star clusters and nebulae, however, will 

 bear fairly high powers. The best power for planetary 

 work is usually about 50 to the inch. Thus a power 

 of 150 should be used with a 3-inch objective. For 

 double stars, a higher power, from 60 to 100 lo the 

 inch, i.e., 180 to 300 on a 3inch, may sometimes be 

 employed. Some stars however are most clearly 

 Soi.tl-. 



April i5tli — i2h. loni. G.^f.T. 



The Planet Ju 



Dr.a'j.'n by the Rei: T/l, 



(S,-c- fai 



After the magnifying powers of the eyepieces have 

 been ascertained, next find the angular diameter ot 

 that portion of the sky visible with each ejepiece, 

 when it is truly focussed. This section of sky is 

 known as the " field of view." To do this, turn the 

 telescope on some star near the celestial equator, such 

 as 5 Orionis, or '/ Virginis, and accurately measure 

 the interval of time elapsing during the passage of the 

 star across the centre of the field of view from Kast to 

 West, then turn time into angular diameter. For 

 instance, if the object takes just two minutes of time 

 to cross the field of view, the angular diameter of 

 that field of view is 30' or half a degree. Minutes or 

 .seconds of time multiplied by 15 give minutes or 

 seconds of arc. 



If possible always use the telescope out of doors, 

 not from a room through a window. If the 

 latter method is sometimes unavoidable, let 

 the end of the telescope be well out of window to 

 avoid the heated air passing from the room. The 

 floor too, is susceptible (_.f every movement, which is 

 communicated to and magnified b_\' the instrument. 

 Never try to look at an object over, or to leeward of 

 a chimney in use, nor, if possible, close to the horizon. 

 It is always best to use a " dew cap," a piece of light 

 tubing about three diameters of the object glass in 



^ra\' 6th — loh. 20m. G.M.T. 



PITER IN 1899. 



toi/orc E. R. Pliilllps. 



ic 2b.) 

 seen with low powers. Practice is the best guide^in 

 these matters. Faint comparisons near to brilliant 

 stars are often best seen in strong twilight, or with a 

 full moon, this masks the glare of the large star. 



In looking at the sun it is safest to employ a solar 

 reflector, which is a surface of glass reflecting only a 

 portion of the light through the ej'epiece and sunshade. 

 This prevents the dark sun-glasses from getting 

 cracked, and sometimes the observer's eye from being 

 destroyed. 



When making observations, always take instarit 

 note of w hat is seen ; for, if an interval is permitted, 

 the memory becomes clouded by uncertainties. 

 Carefully note the date and time of every observation. 

 It is always well, if po.ssible, to supplement written 

 notes by diagrams ot drawings. To do this it is not 

 necessary to have studied drawing. A diagram of, say, 

 Jupiter, would show the position of any markings far 

 better than any amount of written description. With 

 practice comes proficiency. In drawing a planet, or a 

 portion of the moon's surface at the telescope, do not 

 attempt to finish as proceeding. First get rough outlines 

 done, fill in the positions of the spots or shadows, 

 note the time, and then proceed to fill in the finer detail. 

 No one knows the value of even a rough diagram. 

 (To be continued.) 



