520 Notices respecting New Books. 



of a series of zone observations with that instrument, whicli he found 

 it impossible to carry out. In its arrangement the Catalogue is similar 

 to the Catalogues of double stars previously communicated by the 

 author to the Royal Astronomical Society. It consists of nine co- 

 lumns, the first containing the reference-number prefixed to the 

 new double stars ; the second and third the llA and NPD, reduced 

 to 1830; the fourth the angles of position; the fifth the estimated 

 distances ; the sixth the magnitudes ; the seventh notes made at the 

 time of the observation ; the eighth references to other Catalogues 

 with which the stars in this may have been identified, either as single 

 or double stars ; and ninth, the number of the sweep. The Cata- 

 logue occupies seventy-two pages. 



With respect to the angles of position, it is remarked that " they 

 are by no means to be regarded as of equal authority with the results 

 of an equal number of measures taken with the equatorial, for several 

 reasons. 1st. They are for the most part much less deliberately taken. 

 2nd. When taken with care, and when time has been allowed for a 

 repetition of measures, a peculiar bias of judgement seems in some 

 cases to have influenced them, which makes it necessary to be cau- 

 tious in combining them with the equatorial measures of the same 

 stars. This is, no doubt, jjartly owing to the different jDosition of the 

 person and head of the observer at the two instruments — in the one 

 case looking down, in the other uj). To this we must add the dif- 

 ference of inversions due to the optical construction of the two instru- 

 ments ; the achromatic operating a complete, and the reflector only 

 a semi-inversion. Owing to this cause the situation of the line of 

 junction of tha^wo stars observed on the meridian will stand in a 

 diff"erent relation to the optical bias of the eye habitually used for 

 observing, and thus angles obtained with the two instruments will 

 cease to be comparable. As a general principle, indeed, it may be 

 borne in mind that no measured angle of position of a double star, 

 no matter with what instrument taken, can be considered free from 

 bias, unless their line of junction lie in the principal section of the 

 eye, and unless the diurnal motion be either eliminated by clock- 

 work, or by a perfectly equable movement given by hand, a condition 

 tolerably well satisfied in my observations with the equatorial, by 

 long practice in the use of the right ascension handle, but which the 

 construction of the reflector rendered it almost impossible to fulfil 

 with any degree of exactness." — P. 168. 



In respect of the estimated distances of the component stars. Sir 

 John observes, that he considers them in general too small in the 

 closer stars; "they are also affected by the apparent size, neatness 

 of definition, &c. of the stars, and are of course in a very high de- 

 gree vague and precarious, serving for little more than general clas- 

 sification." 



In the Introduction to the Micrometrical Measures with the 7-foot 

 equatorial, some particulars are given respecting the instrument, the 

 building in which it was placed, and the construction of the revolving 

 roof. No alteration was made in the mode of mounting the instru- 

 ment, excepting a change in the length of the polar axis necessary 



