OPTICS. 



57 



The phenomena of the Mirage are 

 most frequently seen in the case of 

 ships when they are just beginning to 

 appear above the visible horizon. Mr. 

 Huddart, Dr.Vince, and Capt. Scoresby, 

 have described various appearances of 

 this kind, of which the following are the 

 most interesting. 



_. On the 1st of 



Fig. 52. Augustf 1798 ^ 



Dr. Vince ob- 

 served atRams- 

 gate, a ship 

 which appear- 

 ed as at A, (Jig. 

 *W 52.), the top- 

 mast being the 

 only part of it 

 that was seen 

 above the ho- 

 rizon. An in- 

 verted image of 

 it was seen at 

 B immediately 

 above the real 

 ship A, and an 

 erect image at 

 C, both of them 

 being complete 

 and well de- 



fined. The sea was distinctly seen be- 

 tween them, as at v w. As the ship rose 

 to the horizon the image C gradually 

 disappeared, and while this was going 

 on the image B descended, but the main- 

 mast of B did not meet the mainmast 

 of A. The two images B, C were per- 

 fectly visible when the whole ship was 

 actually below the horizon. 



While navigating: the Greenland sea 

 on the 28th of June, 1820, Captain 

 Scoresby observed about eighteen or 

 nineteen sail of ships at the distance of 

 from ten to fifteen miles. He saw them 

 from the mast-head, beginning to change 

 their form. One was drawn out, or 

 elongated, in a vertical plane ; another 

 was contracted in the same direction : 

 one had an inverted image immediately 

 above it, as at a, (.fig. 53.), and two, 

 at b and c, had two distinct inverted 

 images in the air: along with these 

 images there appeared images of the ice, 

 as at b and c, in two strata, the highest 

 of which had an altitude of about 15'. 



In a later voyage, performed in 1822, 

 Capt. Scoresby was able to recognise his 

 father s ship, when below the horizon, 

 from the inverted image of it which 

 appeared in the air. " It was," says he, 



Fig. 53. 



" so well defined, that I could distinguish 

 by a telescope every sail, the general 

 ' rig of the ship,' and its particular 

 character ; insomuch, that I confidently 

 pronounced it to be my father's ship, 

 the Fame, which it afterwards proved 

 to be ; though in comparing notes with 

 my father, I found that our relative po- 

 sition, at the time, gave our distance 

 from one another very nearly 30 miles, 

 being about 1 7 miles beyond the hori- 

 zon, and some leagues beyond the limit of 

 direct vision. I was so struck by the pecu- 

 liarity of the circumstance, that I men- 

 tioned it to the officer of the watch, sta- 

 ting my full conviction that iheFame was 

 then cruising in the neighbouring inlet." 

 One of the most curious phenomena 



of this kind was seen by Dr. Vince on 

 the 6th of August, 1806, at 7 p. M. To 

 an observer at Ramsgate, the tops of the 

 four turrets of Dover castle are usu- 

 ally seen over a hill between Rams- 

 gate and Dover. Dr. Vince, however, 

 when at Ramsgate, saw the whole 

 of Dover castle as if it had been 

 brought over and placed on the Rams- 

 gate side of the hill. The image of the 

 castle was so very strong and well de- 

 fined, that the hill itself did not appear 

 through the image. 



In the sandy plains of Egypt the Mi- 

 rage is seen to great advantage : These 

 plains are often interrupted by small 

 eminences, upon which the inhabitants 

 have built their villages, in order to 



