The Planet Mars : a Fragment. 185 



every case, of perspective foreshortening and apparent direction 

 of motion ; so that a succession of views of the Earth, as ob- 

 tained in different positions from the Moon or Yenus, would 

 exhibit a strange degree of dissimilarity, and much difficulty at 

 first in identification. All this holds true with regard to our 

 views of Mars, and even more, from the greater inclination of 

 his axis, amounting to about 28f ° :* and here we find the 

 principal source of the obvious and, at first sight, almost irre- 

 concilable differences in the existing views. To this we must 

 also add other casual sources of deception, such as the mislead- 

 ing of the eye, by the oblique passage of the axis through the 

 field of the telescope, where there is no clockwork motion ; by 

 the effect of phasis, not always duly allowed for, especially 

 when a ready-prepared circle is used for delineation ; and by 

 the fact that the defalcation does not necessarily begin at the 

 white polar spot, but at, or not far from, the apparent vertex. 

 Some of these causes of error, I have had subsequent reason to 

 think, have affected my own sketches, and possibly may have 

 influenced those of other observers. All this considered, we 

 shall cease to wonder at the striking dissimilarity in the re- 

 presentations of Mars taken at different epochs ; and when we 

 have added the equivalent of what astronomers call " personal 

 equation" — the strangely varying judgment formed by different 

 eyes, and occasionally by the same eye under different circum- 

 stances — as well as the inadequacy of many hands in delinea- 

 tion, we shall see reason why even views of the same 

 opposition, taken by different observers, are often more unlike 

 than might have been expected. 



With respect to the drawings accompanying this paper 

 some explanation is requisite. They are the best out of a set of 

 forty-three, extending from September 16 to December 20, 1862; 

 and, being intended to exhibit every portion of the globe then 

 visible in succession, with some repetitions for the illustration 

 of details, they stand in the order of rotation and not of time. 

 The apparent irregularity which will be remarked in the posi- 

 tion and magnitude of the phasis arises from this cause ; its 

 real value having been carefully attended to in each instance.f 

 Equality of size has been adopted for convenience, but in fact 

 the diminution of the disc towards the close, December 20 

 giving only 10"'4 against 21"*8 on September 26, was such as 



* This quantity is subject occasionally to a slight apparent increase, arising 

 from the inclination of the orbit of Mars to our own, which amounts, according 

 to Lindenau's Tables, as cited by Arago, to 1° 51' 0"'2. This value differs Borne- 

 what from that given by Sir William Herschel, who makes the inclination of the 

 axis of Mars to our ecliptic 30 5 18', and to the plane of bis orbit 28° 42'. 



f The same singular and unexplained anomaly of phasis is said by Miidlcr <o 

 occur in Mars as in Venus (see Int. Obs. xvjii. 455), the observed always fallintj 

 short of the computed magnitude. 



