194 Optical Deception. 



70 (p) Ophiuchi (42 of our list) . Kriiger, the great inves- 

 tigator of this star, from further observation prefers a parallax 

 of 0""162, subject to an uncertainty of 0"'0071, inferring a 

 mass in the system 8*1 times greater than that of our sun, and 

 a distance which light would traverse in 20*1 years. 



OPTICAL DECEPTION. 



The evening of July 27, 1863, had been cloudless, but 

 hazy, with flaring and fluttering definition. As night advanced, 

 however, it improved, and towards llh. the discs of the stars 

 came out finely : about this time, in endeavouring to meet with 

 the beautiful triple star e Equulei (which has no place in our 

 list, from the difficulty of finding it from description), I came 

 across a 6 mag. star with a close 8 or 8| mag. companion, 

 at about 1"*5 and 330°, both discs being very nicely defined. 

 Thinking that this might be an object of W. Struve's, which 

 I believed to exist in that quarter, but had never seen, I 

 looked carefully at it, and continued my search, turning to 

 a similar star in the field of the finder, some 2° distant. To 

 my great surprise this had the same appearance. I tried a 

 third; they were all alike. I at once turned the eye-piece 

 (an achromatic combination by Powell and Leland, giving a 

 power of about 300) round its axis ; but the images remained 

 unchanged. I then turned the object-glass one-third round; 

 the companion was still there — I fancied at first somewhat 

 altered in angle, but further examination proved that I was 

 mistaken; and a turn of another third made no difference. 

 A star not far from Al Tair gave the same result, perhaps 

 less distinct. I looked to the west towards Hercules; still 

 something of the same kind came before mo, though not so 

 clearly. I returned to my first three stars ; in each case tho 

 attendant was still perceptible, though much less obvious, and 

 apparently fading away : while occupied among them, I came 

 unexpectedly across my intended object, e Equulei ; and here 

 the beautiful and black division of the close pair at onoe satis- 

 fied me that neither the eye nor tho instrument could be the 

 cause of* Hi is singular deception. I regretted that its short 

 continuance, and toy own haste, prevented me from testing it 

 more Fully on other stars, and with other eye-pieces, but as to 

 what J did Bee, uo donbt remained. During an experience of 

 many years, 1 cfinnol remember a similar illusion, excepting, 

 perhaps Once, with B single star, and even that may be ques- 

 tionable, tt may possibly be referred to somo unusual con- 

 dition of atmosphere, producing, apparently, for a short time, 

 something like double refraction in a vertical plane. Similar 



