a a ee 
J, LeConte on the Silver-Spring in Florida. 7 
other hand, when looking directly downwards, we measure the — 
depth by the angle made at the eye by rays emanating from 
upper and lower submerged objects, this angle being augmented 
by the greater refraction of the former, the depth is exaggerated. 
These physical principles thus afford a satisfactory CEs 
of the peculiar inverted bell-shaped appearance, which t 
presents to an observer floating near its centre. rid iicawise 
explain a fact which strikes the m sual observer: namely, 
that when the boat is advanced toward : spunea shallow 
spot situated at some Smcon it appears to grow deeper as we 
approach the point in questio 
The foregoing is a ge sacra physical explanation of the\ phe- 
nomenon of exaggeration of depth; but the principles of optics 
furnish us with the means of submitting i it to a numerical test, 
and consequently, of showing its adequacy to account for all the 
facts observed. Following out the graphic method, the validity 
of the physical explanation was at first tested by construction. 
While this illustrated in a 
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ing light from the succes- 
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interposed, and EH’ the position of the eye ving 
ponding rays after — by 3 bless 
