S82 Mr Herschel and Mr South's Observations on 



The instruments employed by Mr Herschel and Mr South, 

 were two achromatic telescopes of Jive and seven feet focal 

 length, mounted equatorially. The object-glass of the five 

 feet telescope, has an aperture of 3 J inches, and was made by 

 the late P. and J. Dollond. The power usually employed 

 was 133, but powers of 68, 116, 240, 303, and 381, were oc- 

 casionally used, by double eye-pieces ; and a single lens, with 

 a power of 578, was sometimes applied for the purpose of mi- 

 nute scrutiny. 



The object-glass of the seven feet telescope, which is con- 

 sidered the chef-d'oeuvre of Mr Tully, has a clear aperture 

 oijive inches, and under high magnifying powers it is suppos- 

 ed to be surpassed in distinctness by no refractor in existence. 

 Under favourable circumstances, and with a power of 600, 

 the discs of the two stars of r\ Coronae, and of a Coronae ; 

 of £ Bootis, and of Z, Orionis, are shown perfectly round, and 

 as sharply defined as possible. The power usually employed 

 was 179, but a lower power of 105, and a higher power of 

 273 were occasionally resorted to. 



In observing with these fine instruments, Mr Herschel and 

 Mr South found that the proper degree of illumination was a 

 matter of great consequence, and that it differed in almost each 

 particular star. They found that many very minute stars 

 bore, without extinction, strong degrees of illumination, and 

 were even the better for it, while others apparently brighter were 

 found unable to bear even the slightest extraneous light. 

 This they considered as probably owing to an excess of blue 

 light in the star, forming a contrast* with the ruddy tint of 

 the lamp's illumination, for the most remarkable instances of 

 the phenomena were those in which the small star was de- 

 cidedly of a blue colour. 



For example — 



o Scorpii is much improved by illumination. 



• We are disposed to ascribe this curious phenomenon to the circum- 

 stance of the blue colour of the star being the harmonic colour of the 

 orange yellow tint of the lamp-light- When the retina is impressed with 

 any colour, it is more sensible to weak impressions of its harmonic colour 

 than to any other. — Ed- 



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