374 PROFESSOR C. PIAZZI SMYTH ON THE 



sion seen completely separated, well-formed, and with dark sky between them ; 

 a decided advance on what I have ever witnessed with 7 and 9-inch object-glasses 

 elsewhere by celebrated makers. 



The larger luminous discs also, which the Elchies object-glass gives to stars, 

 on being thrown much out of focus, — and a Lyrge was the one principally experi- 

 mented on, — were pretty regular in their circular formation and strength of illu- 

 mination, though they had a disagreeable feature of colour ; for when the eye- 

 piece was pulled outwards from the focus, the disc was greenish, with a violet 

 border or fringe ; but when pushed inwards therefrom, there appeared a small 

 central disc of violet, with an annular surrounding space of green ; and when set 

 exactly to the focus, the central part of the star was white, or slightly yellowish, 

 astonishingly brilliant, and surrounded with rays that showed an intense violet 

 colour towards their outer ends. 



These effects, as I presume, are chiefly those of the " residual spectrum" of 

 the object-glass, a defect that must exist to some extent in every so-called achro- 

 matic, but varying in character in each case with the judgment of the optician 

 and the nature of the glass at his disposal. In the Elchies object-glass, as the 

 outstanding defect seemed chiefly centered in the violet ray, which contains but 

 a very small portion of light, its consequences may not be of moment in most of 

 the ordinary optical observations, though it may become of importance in any 

 attempt to use the telescope photographically, and did seem to prejudice the eye- 

 estimations of the colours of some few stars. For instance, when a Lyrse entered 

 the field, the eye was so excited by the splendour of the great star, a real first 

 magnitude, and by the glory of its violet halo, that the small companion star, 

 which is only of the eleventh magnitude, and has appeared to me in other tele- 

 scopes of a blue colour, assumed in this one a reddish look, apparently from the 

 complementary effect of the dominant violet of the larger. This case, however, 

 is an extreme one, and when the stars under observation were below the third 

 magnitude, little, if any, of the violet halo was ordinarily noticed. 



A. — 4. Identification of Objects Observed. 



For the large stars, so few in number, their mere names, as every one knows, 

 are quite enough to identify them ; but for the constantly increasing hosts of the 

 small stars that are dealt with by modern astronomers, no system of names has 

 ever proved sufficient. In such case, reference has first been tried to their 

 number's, as they stand in some numerically arranged catalogue ; and if every 

 observer was to refer, or could refer, to one and the same catalogue, that might 

 form a practicable method. But this is not done, and for many various reasons ; 

 recourse being had instead to the numbers in a variety of catalogues, one man 

 making one catalogue his standard of reference, and another, another. 



