522 Notices respecting Neisa Books. 



and Stli, the distance and value attributed ; 9th, the magnitude ; and 

 10th, the date of the observation. The number of stars is 1802. The 

 Catalogue occupies sixteen pages, the accompanying notes eleven 

 pages, and the Synoptic Table seven pages. 



The author then institutes a comparison between the angles of 

 position measured with the 7-foot equatorial and the 20-foot reflector. 

 He states that on comparing the results given by the two instruments, 

 " material discordances will be found in a great many instances, such 

 as cannot be wholly accounted for either by the comparatively greater 

 attention to precision in this respect in the equatorial measures, and 

 the greater time and care bestowed on them, or by the greater num- 

 ber of measures actually taken with the latter instrument. It is true 

 that several cases of great disagreement may be perfectly well ac- 

 counted for in this way, when the closeness of the stars measured, 

 their inequality, and even the difficulty of seeing them at all with 

 the smaller instrument, are taken into account. But on subjecting 

 the matter to a more particular examination, it becomes very evident 

 that these causes alone are not sufficient to account for the discor- 

 dances, and that there exists a systematic bias, from whatever cause 

 arising, which has affected all the 20-foot measurements (as com- 

 pared with the equatorial) — and that diffei-ently according to the dif- 

 ferent direction of the small star with respect to the large one. What 

 is more singular, and indeed, so far as I see, unaccountable, is, that 

 neither the amount nor direction of this bias is the same for all the 

 four quadrants of position ; nor is it even the same in the same direc- 

 tion for positions differing Ijy 1 80"." From a comparison of results 

 given by both instruments he constructs a table of reductions for 

 every 10°, from 0° to 360°, exhibiting what may be called the bias- 

 correction to the positions measured with the 20-foot instrument, by 

 the application of which a much better general accordance, and an 

 even distribution of positive and negative errors were produced. He 

 also remarks, that on examining with care all the cases of compaiison, 

 he found no distinct ground for concluding that either the magnitude 

 of the stars or the angular distances (when well seen and well sepa- 

 rated) exercised any systematic influence on the angles, or on the 

 amount of the bias-correction. 



• The details given on this subject are extremely interesting, from 

 their bearing on certain phsenomena of vision not yet well under- 

 stood. They also show how much caution is necessary in the com- 

 parison of observations of so delicate a kind when made with different 

 instruments, or in a different manner ; and they give a lively idea 

 of the numei'ous difficulties which were encountered in carrying out 

 the undertaking to a successful issue — difficulties of a harassing and 

 perplexing kind, which only a practical astronomer can fully appre- 

 ciate. 



The "Special Remarks" on tlie measures of particular double 

 stars in the two Catalogues, with which the chapter concludes, relate 

 principally to the evidence of angular movement furnished by the 

 comparison of the observations. About twenty-two objects are re- 

 marked on, but we shall confine our extracts to the two which may 



