37 



ADDITIONAL AND CORRECTED ELEMENTS FOR 

 THE TRANSIT OF VENUS, December 9th, 1874. 



WITH NOTES FOR REDUCING THE EFFECTS OF IRRADIATION, 

 AND ERRORS FROM OBSERVATIONS MADE IN THE PLANE 

 OF THE MERIDIAN, Etc., Etc. 



By F. Abbott, F.R.A.S., F.R.M.S. 



Read at a meeting of the Royal Society of Tasmania, 9th of June, 1874. 



h. 



External contact at Ingress 11 



Internal contact at Ingress 12 



Least distance of centres, 13' 46" '7 1 



Internal contact at Egress 3 



External contact at Egress 4 



Mean Time at Hobart Town. 



For direct ( first external contact 50° towards the E. 



image. J first internal contact 44° towards the E. 



Angle from J last internal contact 14° towards the W. 



N. pole of v. last external contact 20° towards the W. 



The Sun's true Semidiameter 16 16 "2 



Venus's true Semidiameter 321 



The Sun's apparent Declination 22 51 29 S. 



The Approximate Meridian Altitude 69 59 



h. m. s. 



The Meridian Passage 11 52 2271 



At Ingress the Sun's Elevation will be 67 



., ,, ,, Retardation 2*8 



At Egress the Sun's Elevation will be 40 



,, ,, ,, Acceleration 7'6 



The exact moment of contact is a little doubtful ; it de- 

 depends on the real size of Venus, as well as on her motions, 

 the size being necessarily a matter of estimation, and the 

 motion of Venus,although well determined, will be, if necessary, 

 corrected by the observations during the transit ; so that the 

 circumstances of the transit will not be appreciably affected, 

 even though there should be a minute or two of error. 



The greatest difficulty to encounter at the coming transit, 

 next to that of a cloudy atmosphere, is irradiation. The eye 

 is by no means a perfect optical instrument. It suffers from 

 spherical aberration ; a scattered luminosity, more or less 

 strong, always surrounding the defined images of luminous 

 objects upon the retina. It is well known that images of 

 objects frequently persist on the retina of the eye, i.e., we 

 continue to see light for a certain period after the light itself 

 has gone out, as in the case of an electric spark, bolides, 

 falling stars, or meteors, &c. Irradiation is a similar action 

 of the retina, in respect of space. The retina does not lose 



