292 Royal Astronomical Society. 



the instrument used at Bedford being, as I understand, the excel- 

 lent 8|-feet achromatic of nearly 6 inches aperture, and conse- 

 quently possessing an illuminating power of between two and three 

 times greater than that of my 5-feet achromatic. Hence it is ma- 

 nifest that some plan must be devised for equalizing the optical 

 powers of the different observers and their instruments, the eye and 

 the telescope being viewed as one compound optical machine. And 

 this being attained, the varying condition of the atmosphere would 

 alone be uncontrollable; for I imagine the differences of the planet's 

 altitude might be very nearly allowed for by a corresponding altera- 

 tion in the aperture or power generally used. 



" 1 would therefore earnestly recommend the adoption of some 

 celestial object as a standard of the optical power to be employed ; 

 and I know of none possessing so many advantages as the minute 

 companion of Polaris. It is readily found, is visible throughout 

 the year, and preserves nearly the same altitude. Let each ob- 

 server then ascertain, by a series of experiments on several clear 

 nights, what is the minimum aperture with which his instrument 

 will enable him, on an average of such nights, to keep this delicate 

 6b]ect just steadily in view. Perhaps it would be proper to fix the 

 power employed at about 80. This power is very usually attached 

 to the 3|-feet achromatic, the aperture of which instrument will, 

 I imagine, to most eyes, admit of a little diminution to bring it to 

 the proposed standard. This telescope is, moreover, not an un- 

 common one among observers; yet I am disposed to regret, that 

 the object I have selected as on the whole the best, is not suffi- 

 ciently visible with the usual 30-inch achromatic. For, the greater 

 the number of observers engaged, the more interesting and useful 

 are the results likely to prove. 



" It appears to me, that by the adoption of this plan, all the ob- 

 servers of these phenomena would be placed as nearly as possible 

 on an equal footing, and every source of uncertainty within our 

 control would be removed. 1 am not, indeed, prepared to suggest 

 precisely the best mode of meeting the differences of the planet's 

 altitude, as seen from different places j but this will not vary enor- 

 mously in the British Isles; and if the plan should meet with the ap- 

 probation of observers within these limits, and by them be sub- 

 jected to a fair trial during the next apparition of Jupiter, further 

 details and improvements may be brought forward, should the me- 

 thod be thought worthy of general adoption. 



" It would, of course, be highly desirable that the same mode of 

 observation should be pursued (in respect at least of owe instrument 

 and observer) at the principal observatories, whose longitude is ac- 

 curately settled, as the results thus obtained would have great in- 

 fluence in determining the question of its practical utility. 



" I need not dwell on the importance of distinctly specifying the 

 atmospheric circumstances under which these observations are made, 

 as otherwise it will be impossible to judge of their comparability. 

 It might also conduce to further improvement of the method, if the 

 planet's hour-angle or estimated altitude were recorded, and it would 

 be convenient were the same denomination of time employed in 



