3H 



SCIENCE-GOSSIP. 



CHAPTERS FOR YOUNG ASTRONOMERS. 



By Frank C. Dennett. 

 CHOICE OF A TELESCOPE. 



In selecting a telescope the chief essential is 

 decidedly quality rather than quantity. A perfect 

 instrument always gives pleasure, whilst an inferior 

 one, notwithstanding the wonders it will bring 

 to view, frequently causes vexation. The first 

 thing to claim attention must necessarily be the 

 optical instrument itself. Telescopes are divided 

 into two classes, refractors and reflectors. Re- 

 fractors have the light condensed by an object- 

 glass, which, of course, should be achromatic, 

 forming at its focus an image of the object to 

 which it is directed, that image being magnified by 

 an eyepiece. Reflectors accomplish the same 



Alt-Azimuth Stand. 

 (Designed by T. Cooke and Sons, Limited. York.) 



result by reflecting the light from a mirror ground 

 to a perfect parabolic curve. These mirrors are 

 now almost always made of glass, the top surface 

 having a thin film of pure silver laid on by a 

 chemical process, which may be renewed when 

 necessary at a small expenditure of money. 



For telescopes up to four inches aperture, as the 

 effective diameter of the objective is called, there 

 is no choice as to the kind of instrument, because 

 reflectors are not usually, or usefully, made less 

 than four and a-half or five inches in diameter. 

 Even so small a telescope as two inches is not to 

 be despised if of good quality. With one no 

 larger, and even of inferior quality, Harding, of 

 Lilienthal, Schrceter's assistant, discovered the 

 minor planet Juno, whilst with an exceptionally 

 good one, Grover has caught the principal division 

 of Saturn's ring. If the objective is good, every 

 increase in size, when the air is steady, means an 

 increase in the defining power as well as the light- 

 grasping power of the telescope. Nearly all so- 

 called achromatic telescopes, however, have this 



drawback : that the colour correction is not perfect, 

 as there is a fringe of outstanding colour around 

 bright objects. This fringe in good instruments 

 appears blue, and in poor ones yellow or red. This 

 is a fault much aggravated in large instruments. 

 Reflectors are, however, free from this defect, 

 there being no chromatic dispersion accompanying 

 reflection. The new object-glasses made by Cooke, 

 of York, under H. D. Taylor's patent, are said to 

 be perfect in this respect, but their price is 

 perhaps somewhat expensive for the ordinary 

 student. If a telescope is good it should separate 

 double stars of about sixth magnitude, whose 

 distance apart is equal to 4"'56 divided by the 

 diameter of the object glass or mirror. Thus 

 a three-inch object-glass should on a favourable 

 night divide a pair whose distance apart is 

 4""56 r 3 = i " - 52. The reason for this is that 

 although stars have no real angular diameter as 

 seen from the earth, yet they have, from the very 

 nature of light, a spurious disc, which, with a 

 one-inch object-glass, is apparently 4"'56 in 

 diameter, and with a two-inch 2"-28. The greater 

 the diameter of the glass, the smaller that of the 

 apparent star disc. As seen with a moderate 

 power, this little disc appears surrounded by one 

 or two thin diffraction rings. 



If possible, choose a telescope of comparatively 

 long focus, for the longer the focus of the 

 objective the higher is the magnifying power with 

 a given eyepiece. Thus if a given object-glass has 

 a focus of thirty-six inches, an eyepiece having a 

 focus of one inch would give a magnifying power 

 of thirty-six diameters. If, however, the focus 

 were fifty-four inches, the amplifying power would 

 be fifty-four with the same eyepiece. This is a 

 factor much felt when the endeavour is made to 

 use high powers. There is rather a tendency of 

 late years to reduce the focal length as much as 

 possible, but this should not be encouraged. The 

 accompanying diagram will readily show the 

 reason. If A B C be a mirror whose centre of 

 curvature is D, parallel rays of light as E A and 

 F C falling upon it will be reflected to a focus at 

 G, just midway between B and D. The rays 

 reflected from the edges of the mirror will have a 

 longer focus than those reflected from near the 

 centre, because a circle drawn around the centre G, 

 at the distance G B, cuts through A G and C C, 

 as is shown by the dotted segment H B I. The 

 practical consequence of this is that the rays from 



the outer zones of the mirror form a larger focal 

 image of the object under examination than do 

 those from the centre. This effect is not felt when 

 stars are being observed, but in the case of the 

 planets it means a blurring of fine details, a 

 delicate black division, say, on Saturn's ring, 

 becoming only like a pencil mark, if indeed it is 

 not blotted out altogether. This has been illus- 

 trated in the case of a mirror because it is so self- 

 evident, but" the result is enhanced in an object- 

 glass. I believe Professor Schaeberle was the 



