St lENi I: GOSSIP. 



I-'; 



CHAll ERS I ' il: VO! KG VSTRONt I 



lt\ I'k \nk ^ I * 

 I i i i SCOPK IPPAF VTVS 



I in p »no(her piece ..t app i 



i voluti /■■ work. This 



in its simplest i ts of a fine slit placed 



.ui achromatic c invex lens. 



This slit is tl > refei 



m ' : i nse flint glass, with the edge parallel 



ing diagram 

 i Fig. ii. t is the slit, b, thi colin 



Fig-, i. Diagr \m op Speci ro i 



prism, and n the ti lescope. E ing the 



the 'ii >unt of tin- ili- 



sufKcient. When very 



lispersion is not, for the use of the amateur, 



required, the direel vision sp is much the 



i i g 



onvenient. These have thres, 

 five, or seven prisms, some "I" dense 

 Mini glass and others "I glass having 

 much less dispersive power, arranged 

 in such a way that the light although 

 i sed, i- m '■ b ml . as when the 

 simple prism is employed. When 

 used with the telescope the object 

 undet ' misaccuratelyfocussed 

 u|i"ti the slit. The image of a star is 

 however only a point, so that iis 

 spectrum Would simply appear as a 

 1 rillianl coloured line, too thin foi 

 its lines to be observed. To overcome this difficult} 

 a cylindrical lens is employed, which changes 

 the figure of the star from a point to a short 

 line. It the line be made to coincide with the slit, 

 thespectrum.il sufficient!} brilliant, may be readily 

 ibserved. 



One of the most convenient forms is the McClean. 

 It can be used either for sun, planets, nebulae 

 or stars, and with any telescope from 3m. aperture 

 upwards. For the stars no slit is required, a con- 



% liinlrio.il lens, within the focus ol the object 

 glass, turns their iri short hrilliani lines, which 



by mean- ol .1 direel vision prism are spn out into 

 a neat spectrum. Fig. 2 shows the arrangement : a 

 being the direct vision prism : B the adaptei 

 screw, and i the cylindrical lens. Thus arranged it 

 screws into the telescope like an ordinary eye] 

 Nebulae ma} receive a preliminary examination with- 

 out a slit, gaseous nebulae retaining their form, whilst 

 tlie stellar lines are spread into a band. One great 



'hat it 

 nut only with 1! 



1 with an altazimuth. An 

 h a colimating lens ma} be placed in 

 id ol the cylindrical lens. I 

 slit being in the focus "I the object glass ; then 

 may be examined. The next diagram ' 1 

 he form "l the arranger 



ipter, c the convex lens, and n the sin with a 

 screw to enable its opening to lie regula ■■!. I 

 dispersion is not however sufficient to permit of the 



b ' 'tion of ih.- 1 >in ilar promii 



though the bright hydrogen lines ind 

 presence may sometimes be seen. 



It will be well to say a word or two about 

 dispersion. The spectrum is realty made up 

 of a multitude of images of the slit, eacn 

 ray, of ever) degrei il refrangil ilit 

 its own image. Notwith indii [thai the slit 1 

 less than one-threchundredlh of an inch open, these 

 tiny images overlap to a gi nt. The.! 



d dispersion is in some measure to overcome 

 this overlapping. The two figures below 'I' 

 and 5) illustrate tin' effect, the upper figure slow, the 

 spectrum spread over m 1 the length of the 



lowei one. I'll.- m - 1'. 1 ' '- of this is 1 ' ; 

 many liner lin me 1 isible, and -' -■ ' nil that 



appeared single to be doubled. When the sun is 

 undei observation great dispersion may be used. 

 which will of course decrease the brilliance ol the 

 light, and so permit of the -Ii; 

 ing "pened sufficiently to allow 

 the tonus, as well as the presence, 

 of the solar prominences to be seen. 

 I "i furthei explanations of thi 



> I cannot do better than 

 refer the reader to the late Ri 

 A. Proctor's little shilling manual 

 on "The Spectroscope and its 

 V\ ork," and [< ihn Browning's "Hi >\\ 

 to Work with the Spectroscope." 



CREEN VIOLET 



RED _ CrtEEN 



Fig-. |. 



Fig 5. 



.1. Hkowimm.s, MacClban's Spectroscope. 



Before leaving the spectroscope I would suggest a 

 1 [uent examination f the northern heavens, on 

 clear nights ; especially if they appear at all light. If 



there be any trace of aurora, its presence w ill be 

 quickly seen by its characteristic bright lines, if the 

 slit be adjusted slightly more open than when used in 

 the daytime. 1 1 the telescope be equatorially 

 mounted, photography may be executed both of the 



sun and m 1. A convenient attachment is supplied 



this purpose by Messrs. Home and Thornthw 

 tl -tellar photography be attempted, a good driving 

 k is an indispensable adjunct. When the stellar 

 photographs are being taken at the Greenwich 

 1 tbservatory, not only is the clockwork going, but the 

 ' is watched in the finder all the time, by an 

 assistant holding an electric controller in his hand 

 speed or retard the clock if necessary. 



I '/',• di (Oitfinu 



