Work for the Telescope. 271 



Huygenian or negative ones are not always perfectly so, in 

 estimating the colours of double stars the image should be kept 

 in the centre of the field : for a similar reason such objects 

 should never be examined near the horizon, when instead of a 

 circular disc, they will show a coloured and flaring spectrum. 



It may be occasionally advantageous to know the diameter 

 of. our field ; this may be roughly estimated from the propor- 

 tion it bears to the diameter of the moon, which may for very 

 ordinary purposes be taken at half a degree. But it will be 

 more satisfactory to note the time which a star near the equator 

 occupies in crossing it ; such as 8 in Orion (the uppermost of 

 the three in the belt) ; or y Yirginis (the method of finding 

 which will be given under No. 10 in the catalogue), especial 

 care being taken to make the star's passage as central as may 

 be. The time thus given will be minutes and seconds of Eight 

 Ascension; these multiplied by 15 (since 15°=1 hour) will be 

 converted into minutes and seconds of space, by which celestial 

 distances are most usually measured. The reason of the em- 

 ployment of an equatorial star (a planet or the moon in the 

 equator will answer equally well) will be at once apparent ; the 

 distance between the meridians, or hour-lines, decreasing con- 

 tinually as they recede from the equator, corresponds less and 

 less with the same number of absolute degrees of space, which 

 are always considered of the same value with those measured on 

 that circle. 



A general idea of the position of the meridian will always 

 be of service ; it may, of course, be obtained from a common 

 compass, allowance being made for the declination of the 

 needle, now amounting to about 21° W. ; or from the gnomon 

 of a good sundial, or from the position of the sun at noon by 

 the dial, that is, uncorrected by the equation of time. 



A few words may be added as to weather. With regard to 

 wide double stars, which may be well seen with a low power, a 

 choice of nights is not very important, the only essential point 

 being that there shall be no haze capable of obscuring the 

 smaller companions. But the matter is entirely altered when we 

 attempt the separation of the closer pairs. Astronomers well 

 know that a high degree of transparency may be combined 

 with a most annoying amount of unsteadiness, and that what is 

 called a brilliant night may hence prove perfectly useless for 

 all delicate investigations : and thus we find HerschelTI. speak- 

 ing of such a night as " the worst possible for vision, though 

 superbly clear." Higher powers becoming necessary as the 

 components are nearer together, every atmospheric defect is 

 magnified in the same proportion. At one time no care in 

 focal adjustment will sharpen up the diffused blot to the sem- 

 blance of a star; at another, the comparatively defined discs 



