Notices respecting New Books. 463 



are not liberal enough to pay for the extra labour, they ought 

 not to be siirj.irised if the makers are Vv'iilltig to stop when the 

 figure is tolerably gco(\, rather than run the risk of destroying a 

 week's nork, by trying to make it a fine one. Get one metal 

 as good as you can, then set to work at another, and when you 

 have made on.e more perli-ct, try to mend the first : tlius, by al- 

 ternately working one after the other, you will at last obtain the 

 *ne plus ultra' of perfection. 



" It is of the utmost consequence to Uie perfection of reflect- 

 ing telescopes, that the mirrors be truly parallel to each other, 

 and also that the centres of them, together with the centres of 

 the eye-glasse-!, be all in one direct line; viz. in the axis of the 

 tube. Indeed, unless these particulars are attended to, the in- 

 strument will prove deffctive and faulty, even though the mir- 

 rors have the most exquisite figure possible given to them. That 

 truly excellect artist, the late ingenious Mr. James Short, always 

 took the greatest care to adjust and centre the metals of his 

 telescopes. If the mirrors are truly centred and adjusted to 

 their best position, a fixed star, when the telescope is piit out of 

 focus, should always appear, in reflecting telescopes, as a truly 

 round circle of fire with a black spot exactly in its centre ; and 

 when the telescope is adjusted to distinct vision, the star should 

 appear, if the telescope is excellent, and the state of the air 

 favourable, exactly rmmd, and totally free from all inadiations, 

 or false rays and glare. Indeed I can assort, from experience, 

 that no object is so proper to determine the excellence of tele- 

 scopes as the fixed stars, as the least irregularity in the figure of 

 the pietals i.u reflecting telescopes. or of the object-glass in achro- 

 inatics, is rendered by them exreedingly coiispieuous by a false 

 glare, and by their not appearing perfectly round." 



To purchasers of telescopes tiie follov/iug useful advice is given: 

 " There is an exUaordinarv and curious fact, with whicii few 

 people are ac([uahited, but is of the first importance every one 

 should be aware of when choosing a telescope, or comparing 

 instruments to ascertain their peculiar powers 3 that when trying 

 astronomical glasses, we should not be satisfied with less thtui 

 three evenings' observation : such is th.e capricious variation of 

 the atmosphere of this country, that some evenings wliich appear 

 extremely fine, when the stars look most brilliant and dazzling to 

 the naked eve, are quite unfit for observation, and our best tele- 

 scopes will not perform. Quiet, serene nights, when there is no 

 moon, are the most favourable. When comparing telescopes, 

 we should take very particular care that the eye-tubes be glassed 

 with the same sort of glass, and tliat they are charged with pre- 

 cisely the same magnifying powers; otherwise the comparison will 

 be in vain : a diffcreuce of even five or ten times in tlie niagui- 



fyiug 



