John Adie, Esq., on the Marine Telescope. 119 



to our sense, from their passage through the denser medium 

 of the water, but from the irregular refractions given to the 

 rays in passing out of the water into the air, caused by the 

 constant ripple or motion of the surface of the water, where 

 that refraction takes place. "Reflections of light from the 

 surface also add to the difficulty ; and before we can with any 

 just hope expect to see the objects distinctly at the bottom, 

 these obstructions must be removed. 



This is done to a very great extent by the use of the in- 

 strument which forms the subject of this* notice; the tube 

 serves to screen the eyes from reflections, and the water 

 being in contact with the glass plate, all ripple is got rid of, so 

 that the spectator, looking down the tube, sees all objects at 

 the bottom, whose reflective powers are able to send off rays 

 of sufficient intensity to be impressed on the retina, after suf- 

 fering the loss of light caused by the absorbing power of the 

 water, which obeys certain fixed laws, proportionate to the 

 depth of water passed through ; for as light passing through' 

 pure sea- water loses half its intensity for each 15 feet through 

 which it passes,* we must, from this cause alone, at a certain 

 depth lose sight of objects of the brightest lustre. The per- 

 fect purity of the water, and its freedom from all muddy 

 particles floating in it, form an important element in the 

 effective use of the water-telescope ; for example, in the 

 Frith of Forth, and similar estuaries, where the influx and 

 reflux of the tide keep particles of mud in constant mo- 

 tion, the instrument is of little or no use ; for these act in 

 exactly the same way in limiting our vision through water, 

 as a fog does through the air : it is therefore only in the 

 pure waters of our northern and western shores that this 

 contrivance is applied with any advantage ; and in such 

 situations we can speak of its powers with confidence. 

 In a trial made with the instrument last autumn on the 

 west coast of Scotland, the bottom was distinctly seen (a 

 white bottom) at a depth of 12 fathoms ; and on a black, 

 rocky bottom, at 5 fathoms under water,, objects were so dis- 

 tinctly seen that the parts of a wreck were taken up — the 



* Leslie's Elements of Nat. Phil., p. 19. 



