11: 



On Measuring the Po^a^er of Telescopic 

 Eyepieces. 



By D. B. Adamson. 



[Read February ist, 1887.] 



In the course of my telescopic observations I have made a 

 point of inquiring of jDarties, when they have been observing the 

 moon, what they considered the aj)parent diameter of that lumi- 

 nary to be when viewed with the naked eye. I have found the 

 variety of opinions on this subject to be very surprising. 



One old gentleman informed me that to him the moon appears 

 to be rather over six feet in diameter ; while another friend says 

 it looks the size of a dinner-plate ; and another tells me it seems 

 about the size of a saucer ; while another gives the size of an 

 onion as the moon's apparent diameter. Now these different 

 answers, although so various, might each and all of them be cor- 

 rect, as the apparent magnitude of any object depends entirely 

 on the distance at which that object is viewed ; so that if a six- 

 feet rod or the dinner-plate, the saucer, or the onion, were placed 

 between the moon and the eye of the observer, and each at such 

 a distance that it would exactly cover that luminary from view, 

 then their apparent magnitudes would, of course, be equal. But 

 for this purpose those articles would require to be placed at such 

 a distance from the observer that they could not be distinctly 

 seen by the unassisted eye. 



When a person of normal eyesight wishes to examine any 

 small object minutely he holds it at a distance of about eight 

 inches from his eye, and we find, if we hold a threepenny-piece at 

 this distance, it far more than covers the whole of the moon's 

 disc, and I find Avlien I hold the blade of my penknife at this 

 distance, it quite eclipses the moon. Now the width of that 

 blade is not six feet, but only about three-sixteenths of an inch. 



In showing jDersons the heavenly bodies in a telescope, they are 

 very often sceptical about the magnifying powers, and when they 

 are informed that the power they are using gives, say, one 

 hundred diameters, they will say, " Certainly the object is ex- 

 ceedingly sharp and distinct, and seems so near that you feel as 

 if you could put your finger on it. But surely it is not magni- 

 fied so much ?" 



To convince anyone of the magnifying power, if the instrument 

 be a " refractor," we have only to tell him to look into the tele- 



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