Royal Astronomical Society, 29 1 



On the Adoption of a Standard of Optical Power by Observers. 

 By the Rev. W. R. Dawes. 



" Almost from the first invention of telescopes to the present 

 day, the eclipses of Jupiter s satellites have afforded matter of in- 

 terest to the astronomer, and to all who are anxious for the im- 

 provement of geographical science. Unhappily, however, notwith- 

 standing all which has been done to improve this most simple me- 

 thod of determining differences of terrestrial longitude, there are 

 few observations whose results are less satisfactory. Nor will this ex- 

 cite surprise, if it be considered, that the instant when any of these 

 phaenomena is observed is greatly influenced by the circumstances 

 under which it is witnessed. When observed at different places, 

 the results may be affected by diverse states of the atmosphere, 

 and altitudes of the planet ; besides the varieties exiscing in the in- 

 struments employed, and the eyes which use them. But though it 

 is beyond our power to control the state of the atmosphere, or to 

 alter the situation of the planet, we may, I conceive, do much to 

 remove the enormous discrepancies arising from the differences in 

 the optical power and acuteness which are brought to bear on this 

 interesting class of phaenomena. 



" It might be supposed, perhaps, that a sufficient agreement of 

 optical power might be obtained by fixing on a certain aperture, 

 and a suitable magnifying power, as a standard to be adopted by 

 all observers who are desirous of determining differences of longi- 

 tude by means of these eclipses. And, no doubt, a much nearer 

 approach to what is desirable would be thus obtained, than can be 

 expected while some are employing telescopes possessed of im- 

 mensely greater illuminating power than those used by others for 

 the same purpose. However important in other respects observa- 

 tions may be, made in well determined stations, with large instru- 

 ments, they are obviously not comparable with the results obtained 

 by such instruments as are usually possessed by private individuals 

 of astronomical taste, and to whom this species of observation is 

 commonly very interesting, and would be more so were its results 

 less unsatisfactory. Yet, were we to agree to cut down the larger 

 instruments to a certain aperture, we should still leave two sources 

 of disagreement unprovided against, which however are within our 

 control. First, the instruments thus apparently placed on a foot- 

 ing would differ much in excellence, and consequently in the time 

 when they would render an immerging or emerging satellite only 

 just visible. Secondly, even could this point be adjusted, there 

 would remain the very great variety which exists in different indi- 

 viduals in respect of acuteness of vision. This 1 believe to be much 

 greater than is generally supposed ; for I know it to be such, in 

 some instances, as to render the 42-inch achromatic telescope as 

 efficient an instrument to one observer in discerning minute points 

 of light, as a 5-feet achromatic is to another j each instrument 

 having its usual aperture, and being of corresponding excellence. 

 In further proof of this, I may advert to the fact, that in every in- 

 stance in which Captain Smyth and myself have witnessed the same 

 emersion, the satellite appears to have been first discerned by me ; 



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