TISION UNDEE WATER. 41 



Reply hy TV. N, 



rp 



-»L HE proper answer to the preceding letter will consist 

 in relating a few experiments which I have just made, 

 along with two of my friends. 



We took neither Oriental nor Batavian waters, but a 

 portion from that venerable stream in which my corres- Experiment, 

 pondent and his schoolfellows so successfully exercised ^^^f^ ^^^^^ deep] 

 their sight. With this we filled a glass cylindrical vessel, was filled with 

 2 feet high and 1 foot wide, standing upon a white (pa- Thames water. 

 per) ground. Two pieces of black lead, sawed square, 

 were put into the water, one of which is one inch and a 

 half wide, and the other only half an inch. Both these 

 pieces were very distinctly seen by the eye above the 

 water, but they were not visible to- any of us when we 

 looked at them with the face plunged beneath the sur- 

 face. 



The larger piece exhibited a darkish hazy appearance, A square of an 



which was very faint and not at all likely to have been ^"'^^ ^"^ ^J}^^^ 



1 ./. 1 . , 1 t^., 1. ■, scarcely dis- 



noticed, if the attention had not been steadily directed to cemable ; a 



its known place. The smaller piece did not perceptibly smaller square 



aJSect the white ground. 



A square piece of cork was fixed to a bended rod, so Cork square, of 

 that it could be plunged to different depths in the water. bk'unksTvery 

 At the bottom of the vessel, it was quite invisible to the near. 

 immersed eye. I could just see that it was square when 

 at the distance of six inches. Others were not satisfied 

 as to its figure, but at a less distance. 



A buckle and an egg were also tried, the former was A buckle and 

 invisible, and the egg (upon a black ground) was very *^ ^SS* 

 hazy, confused and indistinct, so that we did not think 

 it could have been found by sight, by a diver. 



Various convex lenses were applied to the eye, under a convex lens 

 ivater, in order to bring the rays to their proper focus on ^f half an inch 

 the retina. The lens which answered our purpose, is a ^he eye to see 

 double convex crystal glass of half an inch focus in the air distinctly, 

 (but two inches from under water) when this lens was 

 held close to one of the immersed eyes, the other being 

 shut, the objects at the bottom of the water were distinct- 

 ly seen. I clearly observed the saw marks on the black 



VoL» XV — Sept. 1806, G lead 



