SCIENCE- G OSSIP. 



line between their centres. "What explanation can 

 be given of the fact of the moon being impelled 

 at an enormous speed in front of the earth's 

 path against the force of gravity ] — John Clark, 

 Blrmingliam. 



The real daily motions of the earth and moon 

 are about 1,600.000 miles and 55,000 miles re- 

 spectively. Mr. Clark's difficulty seems to me met 

 by the fact that we must not regard the path 

 round the sun as that of the earth, but as that of 

 the earth and moon. The true orbit of the system 

 is that of the common centre of gravity of the two 

 bodies. Both the centre of the earth and the 

 moon travel along zig-zag courses, being now 

 accelerated, now retarded, in their motions as 

 seen from the centre of the system, which, by 

 the way, whilst it is within the sun, is not the 

 centre of the sun. The sun's direct attraction on 

 the moon is more than twice that of our earth. 

 The lateral motion of the moon is so slight that 

 she is alwavs travelling along a path concave to 

 the sun.— F. C. B. 



Spectroscopes. — These instruments are of two 

 classes, which are alike in having a fine slit 

 admitting the light it is sought to analyse, and a 

 collimating lens to render the rays of light from 

 the slit parallel. After being spread out into a 

 spectrum it is usual, except in pocket instruments, 

 to have a small telescope with an astronomical eye- 

 piece to observe the lines, if any, which are shown. 

 The difference of the two classes is the means used 

 to effect the dispersion of the spectrum. In one 

 class it is a prism, or prisms, employed, and the 

 light rays of the various wave-lengths are refracted 

 to a varying extent, the result being the beautiful 

 spectrum with which most of us are familiar — if 

 not in practice, at least in book illustrations. In 

 the other class a diffraction plate takes the place 

 of the prisms, having upon it a series of parallel 

 lines, some thousands to the inch. An expensive 



Rowland plate, ruled at the Johns Hopkins Univer- 

 sity, may be employed, or a copy of the same pre- 

 pared by Mr. Thos. Thorp, of Whitefield, Manchester. 

 These diffraction gratings give greater dispersion 

 of the red rays than do prisms giving a spectrum 

 of the same length. This is shown on the diagram, 

 where A represents the position of the principal 

 Frauenhofer lines in the prismatic spectrum, and B 

 the same lines in the diffraction spectrum. The 

 spectra given by the gratings are known a> 

 •• Normal," because the exact wave-length of every 

 line may be measured ; whilst measures made with 

 the prismatic spectrum are simply arbitrary, and 

 differ with every variety of glass used. A prismatic 

 spectroscope needs more prisms to produce greater 

 dispersion, but with the diffraction grating a spec- 

 trum of a different order, obtained by a slight motion 

 of the grating, gives the dispersion required. A 

 Thorp transmission grating, 14,500 lines to the 

 inch, in my possession, without any observing tele- 

 scope separates quite widely the D lines in the 

 solar spectrum. A plate of this description is 

 valued at only a few shillings : whilst prisms, to 

 produce equal dispersion, would cost pounds. A 

 solar spectroscope on this plan, though lighter than 



a prismatic one. costs less than a quarter of the 

 money — a consideration to many people. A grating 

 used without a slit, but with a cylindrical lens, 

 will show the lines in the spectra of the brightest 

 stars even with an instrument so small as the 

 3-inch Wray Scien'CE-Gossip telescope. Mr. Thorp 

 has a form of grating mounted on a prism which 

 gives a very brilliant direct-vision spectrum of 

 great dispersion, separating the D lines well. This 

 latter is useful in spectroscopes of the miniature 

 class where a maximum of dispersion is desirable 

 combined with a minimum of bulk — a class of 

 instrument we have found most useful at times, 

 especially for studying the spectra of Aurorae, or 

 showing the true character of a brightening in the 

 northern sky. 



CHAPTERS FOR YOUNG ASTRONOMERS. 



Br FiiANK C. Dexxett. 



(Continued from p. 281.) 



JUPITER. 



Sometimes most rapid changes have been noticed 

 on Jupiter. With the achromatic telescope at 

 the Dearhorn observatory its greatest diameter has 

 been found to be not always identical with the 

 equator, and to vary in direction some five degrees 

 in the course of a week. 



The spectrum shows the presence of watery 

 vapour in the atmosphere of Jupiter, which how- 

 ever does not appear to be quite identical with our 

 own. This will be seen by the somewhat rough 

 copy taken from Sir William Huggins' map from the 

 "Philosophical Transactions of the Royal Society " 

 for May 1864. 



Low 



Sun 



1 :"■" "" !■ l i jj 



J £iti — rr i i' J i< 



Sky 

 Jupiter 



Fig. 6. Spectrum of Jupitek. 



Although Jupiter does not exhibit well-marked 

 phases like Mars, if examined when near quadra- 

 ture it shows a very evident shading-off of the 

 limb farthest from the sun. 



The first known observation of Jupiter was made 

 at Alexandria about 6 a.m. on September 4th, 

 B.C. 240, when it occulted the star 5 Cancri. 

 Castor was also occulted on November 22nd. 1716, 

 as witnessed by Pound. 



It is thought to be very probable that Jupiter, 

 instead of being a dark body like the earth or 

 moon, gives out a certain amount of light to its 

 satellites. 



Most useful work can be done in noting the 

 exact time at which the various markings, bright 

 spots, dark spots, belt ends, or broadenings, cross 

 the central meridian. Calculate the longitude, 

 then the exact relative motions on the planet's sur- 

 face can be learnt when observation is compared 

 with observation. 



{To he continued.) 



Total Eclipse of the Sun in next May 

 should be seen well in Sumatra, where Professor 

 E. Barnard intends to go. An expedition is also 

 to be sent from the University of California at the 

 expense of Mr. W. H. Crocker of San Francisco. 



