NATURAL PHILOSOPHY. 143 



amination of mica minerals, and as it is desirable that the mineralogical 

 arrangement and classification of this species should be founded upon 

 their optical properties, it becomes important that mineralogists should 

 have in connection with a goniometer the means of readily determining 

 the variety of such micas as come under their observation. Mr. Blake 

 has used his instrument in this way, and gave to the Association the 

 apparent angular inclination of the optical or resultant axes of several 

 American micas. He is still engaged upon the investigation. Editors. 



DESCRIPTION OF THE MARINE OR WATER TELESCOPE. 



WE find in Jameson's Philosophical Journal, for July, a detailed de- 

 scription of the water-telescope, the use of which among the Norwe- 

 gians was noticed in the Annual of Scientific Discovery for 1850. It 

 consists of a tube of metal or wood, of a convenient length, to enable a 

 person looking over the gunnel of a boat to rest the head on the one 

 end, while the other is below the surface of the water ; the upper end 

 is so formed that the head may rest on it, both eyes seeing freely into 

 the tube. In the lower end is fixed (water-tight) a plate of glass, 

 which when used is to be kept under the surface of the water. A con- 

 venient size for the instrument is to make the length three feet, and the 

 mouth, where the face is applied, of an irregular oval form, that both 

 eyes may see freely into the tube, with an indentation on one side to 

 facilitate breathing, so that the moisture of the breath may not be 

 thrown inside of the tube. Handles for holding the instrument are to 

 be affixed to each side. The glass at the extremity of the tube should 

 be surrounded with a rim of lead, one fourth of an inch thick and pro- 

 jecting forward 3 inches, so as to form a continuation of the tube. The 

 weight of the lead serves both to sink the tube and in some measure to 

 protect the glass. Holes should be made at the junction of the rim 

 with the glass, in order to allow the air to escape and bring the water 

 in contact with the glass. 



The reason why we so seldom see the bottom of the sea or of a pure 

 lake, where the depth is not beyond the powers of natural vision, is 

 not that the rays of light reflected from the objects at the bottom are so 

 feeble as to be imperceptible to our senses, from their passage through 

 the denser medium of the w r ater, 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 refrac- 

 tion takes place. Reflections of light from the surface also add to the 

 difficulty, and before we can expect to see objects distinctly at the bot- 

 tom, these obstructions must be removed. This is done to a very great 

 extent by means of the water telescope ; 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 refractive powers are able to 

 send off rays of sufficient intensity to be impressed upon the retina, 

 after suffering the loss of light caused by the absorbing power of the 

 w r ater, which obeys certain fixed laws proportionate to the depth of 

 water passed through ; for as the light passing through pure sea-water 



