384 ASTRONOMICAL OBSERVATIONS MADE AT [47 



Professor Challis has constructed Tables similar to Bessel's, and specially 

 adapted to facilitate the calculation of refraction for this Observatory. 

 These tables, together with the precepts for their use, are printed at 

 the end of this Introduction. By their means the total refractions in 

 K.A. and N.P.D. may be found if required, as well as the differential 

 refractions spoken of above. 



When the Comet is compared with a Star in N.P.D. only, with the 

 Clock going, it is usual to bisect the two objects alternately, beginning 

 and ending with the Star. 



The micrometer readings for the Star will vary in consequence of the 

 variation of the refraction in N.P.D. From two consecutive readings, the 

 reading corresponding to the intermediate time of bisection of the Comet 

 may be deduced on the supposition that the readings varv proportionally 

 to the time, and the result may be treated as if the bisections of the 

 Comet and the Star had been simultaneous. 



In this case, if Ao. and AS denote the approximate excesses of the 

 Comet's R.A. and N.P.D. respectively, we have 



Excess of the Comet's refraction in N.P.D. 



15& k 



= - ? sin <j> cos <f) sin h x Aa + --[I cos" <h sin 2 fi\ x AS, 

 cos 2 cos j L 



where the other symbols have the same signification as before. 



For the observations of Mars made in 1862, for the purpose of de- 

 termining the Sun's Parallax, the micrometer-wire was adjusted so as to 

 be at right angles to the apparent diurnal path of a star across the 

 field of view. 



In this case, we have 



True excess of the planet's R.A. above that of the star 



. ., 2k t&nZQ sin(S'-P^) 



= apparent excess of planet s R.A. --- - /s> ^^ .- = . -- L__-iZ x AS, 



' 15 sin 8' 



employing the same notation as before. 



The ninth and tenth columns respectively contain the excesses of the 

 Comet's RA. and N.P.D. above the R.A. and N.P.D. of the Star, as 

 given by the observations when cleared from the effects of refraction. 



In the same columns are placed the coefficients for finding the Comet's 

 Parallax in R.A. and N.P.D. respectively. From the nature of the case, 

 no confusion can arise from placing two such different quantities in the 

 same column, half of the space in which would otherwise be wasted. 



