294 sun's distance. 



distinctness of vision he ascribed principally to the use of a graduated 

 dark glass, constructed under his direction by Mr. Simms. It consists 

 of a long wedge of red glass and a long wedge of green glass, their 

 edges turned the same way, combined with an equivalent wedge of 

 colorless glass, its edge turned the opposite way. Nobody would 

 suppose without trial how fastidious the eye is as to the proper intensity 

 of shade, and how distinctly, when intent on clear vision, it rejects a 

 shade in the most trifling degree lighter or darker. He thought it 

 highly important that such shades should be used for observing the 

 transits of Venus. It is desirable also that the color left by the shade- 

 glass should be agreeable to the observer's eye. 



Still there is one caution which must not be put out of sight. The 

 selection of places depends entirely upon the portion of the Earth 

 which is illuminated at the times of ingress and egress ; and if the 

 tables of the movements of Venus are erroneous in 1882 to the amount 

 of an hour's motion, the illuminated face of the Earth will be altered 

 to the amount of two or three hours' rotation of the Earth, and the 

 selections of stations may be totally changed. It is therefore most 

 important that the tables of Venus should be thoroughly examined, 

 and where necessary rectified. A great mass of observations of Venus 

 exist, already reduced so far as to require only the very last step of 

 substitution of errors of planetary elements. The Astronomer Royal 

 referred particularly to the Greenwich Planetary Reductions from 

 1750 to 1830, to the reduction of certain Cambridge observations, to 

 the reduction (in the annual Greenwich volume) of the Greenwich 

 observations down to the present time, and to the discussion of some 

 of the Greenwich observations by Mr. Main and Mr. Glaisher. And 

 he took the opportunity of expressing his opinion that fifty pounds 

 spent on calculations with an object like this would confer much 

 greater benefit on astronomy than a thousand pounds employed in the 

 foundation and equipment of an observatory. 



On viewing the expense and the risk of the determinations of the 

 Sun's distance by transits of Venus, as well as the distance of time, 

 which must necessarily place them beyond the knowledge of many 

 observers of the present day, it appears natural to consider whether 

 other methods cannot be used, less stringent as to the moment of ob- 

 servation, requiring less co-operation of observations, and occurring 

 at an earlier time. Such are the direct determinations of the parallaxes 

 of Venus and of Mars, when near to the Earth, by simultaneous 

 observations at northern and southern stations, as in figure 1, or by 

 successive observations at the same observatory when it is brought to 

 different positions by the Earth's rotation, as in figure 6. 



Venus cannot be compared with stars on the meridian. She may 

 be compared with stars in extra-meridional observations before sun- 

 rise or after sunset, but she is then uncomfortably bright, and rarely 

 well defined; and she has only one illuminated limb admitting of ob- 

 servation, and therefore in the comparison of observations made at 

 different stations there is great risk of error from difference in the 

 estimation of her semi-diameter. Moreover, she does not remain long 

 in the position nearest the Earth, and the nearer she is the more con- 



