I 26 



NA TURE 



{Dec. 13, 1877 



appearance which necessitates the use of the largest 

 apertures in the observation of close double stars, as the 

 S'ze of the star's disk varies, roughly speaking, in the 

 inverse ratio of the aperture. 



In our climate, which is not so bad as some would 

 make it, a 6- to an 8-inch glass is doubtless the size which 

 will be found the most constantly useful ; larger apertures 

 being frequently not only useless, but hurtful. Still, 4 or 

 3I inches are apertures by all means to be encouraged ; 

 and by object-glasses of these S'ze5, made, of course, by 

 the best makers, views of the sun, moon, planets, and 

 double stars, may be obtained, sufficien ly striking to set 

 many seriously to work as amateur observers, and with a 

 prospect of securing good, useful results. 



Observations should always be commenced with the 

 lowest power, gradually increasing it until the limit of the 

 aperture, or of the atmospheric condition at the time, is 

 reached. The former may be taken as equal to the 

 number of hundredths of inches which the diameter of 

 the object-glass contains. Thus, a 3f-inch object-glass, 

 if really good, should bear a power of 375 on double stars 

 where light is no object ; the planets, the moon, &c., will 

 be best observed with a much lower power. 



Fig. 7. — Appearance of diffraction 

 rings round a star when the ob- 

 ject-glass is properly adjusted. 



Fjg. 8. — Aopearance of same object 

 when object-glass is out of adjust- 

 ment. 



Care should be taken that the object-glass is properly 

 adjusted. And we may here repeat that this may be done 

 by observing the image of a large star out of focus. If 

 the light be not equally distributed over the image, or the 

 diffraction rings are not circular, the screws of the cell 

 should be carefully loosened, and that part of the cell 

 towards which the rings are thrown very gently tapped 

 with wood, to force it towards the eyepiece, or the same 

 purpose may be effected by means of the set-screws always 

 present on large telescopes, until perfectly equal illumina- 

 tion is arrived at. This, however, should only be done in 

 extreme cases ; it is here especially desirable that we 

 should let well alone. In the case of mirrors, instructions 

 for adjustment are generally given by the maker. 



The convenient altitude at which Orion culminates in 

 these latitudes renders it particularly eligible for observa- 

 tion ; and during the first months of the year, our readers 

 who would test their telescopes will do well not to lose 

 the opportunity of trying the progressively difficult tests, 

 both of illuminating and separating power, afforded by its 

 various double and multiple systems, which are collected 

 together in such a circumscribed region of the heavens 

 that no extensive movement of their instruments — an 

 important point in extreme cases — will be necessary. 



Beginning with 8, the upper of the, three stars which 

 form the belt, the two components will be visible in 

 almost any instrument which may be used for seeing 

 them, being of the second and seventh magnitudes, and 

 well separated. The companion to /3, though of the same 

 magnitude as that to S, is much more difficult to observe, 

 in consequence of its proximity to its bright primary, a 

 first magnitude star. Quaint old Kitchener, in his work 

 on telescopes, mentions that the companion to Rigel has 

 been seen with an object-glass of 2|-inch aperture; 

 it should be seen, at all events, with a 3-inch. . The 

 bottom star in the belt is a capital test both of the 



dividing and space-penetrating power, as the two bright 

 stars of the second and sixth magnitudes, of which the 

 close double is composed, are exactly 2|" apart, while 

 there is a companion to one of these components of the 

 twelfth magnitude about I" distant. The small star 

 below, which the late Admiral Smyth, in his charming 

 book, " The Celestial Cycle," mentions as a test for his 

 object-glass of 59 inches in diameter, is now plainly to 

 be seen in a 3|. The colours of this pair have been 

 variously stated. 



Thit either our modern opticians contrive to admit 

 more light by means of a superior polish imported to the 

 surfaces of the object-glass, or that the stars themselves 

 are becoming brighter, is again evidenced by the point of 

 light, preceding one of the brightest stars in the system 

 composing or. This little twinkler is now always to be 

 s?en in a 3|-inch, while the same authority we have before 

 quoted — Admiral Smyth — speaks of it as being of very 

 difficult vision in his instrument of much larger dimensions. 

 In this very beautiful compound system there are no less 

 than seven principal starr ; and there are several other 

 faint ones in the field. The upper very faint companion 

 of X is a delicate test for a 3|-inch, which aperture, how- 

 ever, will readily divide the closer double of the principal 

 S'ars which are about 5" apart. 



These objects, with the exception of t, have been given 

 more to test the space-penetrating than the dividing 

 power ; the telescope's action on 52 Orionis will at once 

 decide this latter quality. This star, just visible to the 

 naked eye on a fine night, to the right of a line joining 

 a and S, is a very close double. The components of the 

 sixth mignitude are separated by less than two seconds 

 of arc, and the glass which shows a good wide black 

 divisioi between them, free from all stray light, the 

 spurious disc being perfectly round, and not too large, 

 is by no means to be despised. 



Then, ag^in, we have a capital test object in the great 

 nebula to which reference has already been made. 



The star to which we wish to call especial attention is 

 situate (see Fig. 4) opposite the bottom of the " fauces," 

 the name given to the indentation which gives rise to the 

 appearance of the " fish's mouth." This object, which 

 has been designated the "trapezium," from the figure 

 formed by its principal components, consists, in fact, of 

 six stars, the fifth and sixth (y' and a) being excessively 

 faint. Our previous remark, relative to the increased 

 brightness of the stars, applies here with great force ; for 

 the fifth escaped the gaze of the elder Herschel, armed 

 with his powerful instruments, and was not discovered 

 till 1826, by Struve, who, in his turn, missed the sixth 

 star, which, as well as the fifth, has been seen in modern 

 achromatics of such small size as to make all comparison 

 with the giant telescopes used by these astronomers 

 ridiculous. 



Sir John Herschel has rated y' and d of the twelfth and 

 fourteenth magnitudes— the latter requires a high power 

 to observe it, by reason of its proximity to a. Both these 

 stars have been seen in an ordinary 5-foot achromatic, 

 by Cooke, of 3f-inches aperture, a fact speaking volumes 

 for the perfection of surface and polish attained by our 

 modern opticians. 



Let us now try to form some idea of the perfection of 

 the modern object-glass. We will take a telescope of 

 eight inches aperture, and ten feet fo:al length. Suppose 

 vye observe a close double star, such as | Ursse, then the 

 images of these two stars will be brought to a focus side 

 by side, as we have previously explained, and the distance 

 by which they will be separated will be dependent on the 

 focal length of the object-glass. 



If we take a telescope ten feet long and look at two 

 stars 1° apart, the angle will be 1° ; and at ten feet off the 

 distance between the two images will be something like 

 2j'j5 inches, and therefore, if the angle be a second, the 

 hues will be the girVo^^ P^""^ ^^ ^^^^j o'^ about Yv^oa^h part 



