Colours of Stars, 439 



orange ; and the same holds good with regard to the secondary 

 or mixed colours. If, therefore, a telescope has, as usual, a 

 fringe of blue or purple " outstanding," as it is termed, when in 

 focus, the image of a white star will be, in proportion to the 

 strength of the fringe, slightly stained with the comple- 

 mentary orange or yellow; such, for example, was the case 

 with the noble achromatic at Dorpat — one of the first instances, 

 if not the first, of a combination of magnitude with perfection, 

 and with which W. Struve's great catalogue of double stars 

 was formed ; what were considered moderate powers in this 

 instrument, 254 and 420, were preferable to 532 and 682, as 

 the latter gave a yellowish tinge. And such is probably 

 especially the case with other productions of the Munich 

 Optical Institute, whose glasses are said to be characterized, 

 notwithstanding their fine definition, by a great deal of out- 

 standing blue. In addition to this never-failing source of 

 discoloration, the fact that different object-glasses may not 

 possess the same intensity, or precisely the same hue, of out- 

 standing fringe, may somewhat vary the colour of their 

 respective focal images ; the material, too, of the older glasses 

 would exercise an influence, the crown glass formerly em- 

 ployed having a strong green cast, from which the modern 

 plate is comparatively free. This coloured fringe is entirely 

 absent in reflecting telescopes, whence their focal image, when 

 equally sharp with that of the achromatic, is more pleasant to 

 the eye ; but the Gregorian construction was apt to exhibit a 

 " smoky " tinge ; and though Newtonians, for some reason 

 which does not plainly appear, are less subject to this, it may 

 be readily induced by using too much copper in forming the 

 speculum metal, or by a slight amount of tarnish. From all 

 these defects, it is pleasant to know that the silvered specula, 

 now coming into use,* are quite exempt, and nothing can 

 surpass the intense purity of their reflection so long as they 

 retain their original brilliancy, which, there is every reason to 

 believe, may, with due care, be preserved for many years, 

 and may be always perfectly restored with great facility. 

 "We must bear in mind, too, that even the most faithful focal 

 picture may receive a tinge from being viewed through a 

 defective eye-piece, though this is not much to be appre- 

 hended, provided the object is kept in the centre of the field. 



* The following very interesting announcement is taken from a Paris news- 

 paper, of May 27 :— " L' Association pour l'Avancement de l'Astronomie et de la 



Met^orologie tiendra une Seance Generate le 3 Juin, a l'Observatoire, a. trois 



heures de l'apres-midi. Le president exposera le but de 1' Association. Le 

 nouveau grand telescope de m 80, monlc equatorialement, sera explique ; le 

 precede d'argenture du miroir sera experiment e." The fraction of a " metre " 

 here given is equal to 2 feet 7 J inches, English measure. 



