380 Scientific Proceedings, Royal Dublin Society. 



The apparent diameter of this planet varies from about 3" at 

 conjunction to about 26" at opposition. On the morning of the 

 19th its diameter was 4"" 8 ; so that the amount of light we 

 received from it was only about the 23-th of what we receive at 

 opposition. 



At this time too, before daylight became too strong, the 

 colours were very strikingly contrasted. That of Mars was a 

 ruddy orange, that of Saturn a pale yellow, while Venus shone 

 like polished silver. 



Very soon after the planets rose it became evident from their 

 distance apart, and the sluggish pace at which they approached, 

 that no apparent occultation would take place before daylight 

 obliterated the group, and the greater part of this interval was 

 utilized in taking micrometric measures of the distance between 

 the planets with the Pistor and Martins micrometer, described in 

 Part 1. of the Dunsink Observations. The first three measures 

 were made by me, the remainder being obtained by Sir E. Ball, 

 each of us in turn writing down the other's observations so as to 

 save time, as dawn was rapidly coming on. For this reason the 

 observations were made in a somewhat more hurried manner than 

 we could have wished; and it is possible that the number of 

 complete revolutions of the screw may be erroneous, as the scale 

 on which the number of revolutions is read is exceedingly small, 

 and an error of a complete revolution is very easily made. But 

 it is most unlikely that an error of this sort remains in the results 

 given below. I have, however, given the actual readings, as well 

 as the results derived from them, lest such an error should exist. 



We first measured the distance between the inner limbs, then 

 the distance between the outer limbs, and then again that between 

 the inner limbs. By applying the value 9"" 65 for the sum of the 

 semi-diameters, we obtain the results in the last column for the 

 distance between the centres. It is, of course, possible to obtain 

 two measures of the distance from the series without assuming a 

 knowledge of the semi- diameters ; for if we take the mean of the 

 first and third measures, and again the mean of this and the 

 second, we obtain the distance between the centres at 17 h 15 m, 7 

 Gr.M.T. Similarly Sir Robert Ball's first three measures give an 

 independent result. 



The atmosphere was unsteady, and the discs very ill defined 



