Altitudes at Sea when the Horizon is Invisible. 99 



quantity for all celestial objects, but it varies with the dis- 

 tance of terrestial objects. As considerable care and applica- 

 tion are necessary, in order to acquire correctness and facili- 

 ty in the practice of this method of observation, it will be pro- 

 per, when practicable, that the observer should accustom 

 himself to take observations by this method on shore, before 

 he proceeds to sea. 



Matthew Adam. 

 Inverness Academy, ]9th Nov. 1825. 



N. B. — The nautical eye tube, formerly tried at sea by Mr 

 Adam, being more complex in its eonstruction, was less easi- 

 ly used, and at the same time less susceptible of accuracy, 

 than the one above described. Yet, upon reference to a list 

 of 199 altitudes of the sun, moon, and stars, taken with it by 

 Mr Adam on board H. M. sloop Clio, in the North Sea, in 

 October 1823, it appears that the mean difference of these al- 

 titudes, taken by the eye tube, from those taken at the same 

 time, in the ordinary way, by the officers of the Clio, was less 

 than one minute and a half. 



Several of the eye tubes, above described, are now about 

 to be tried under the direction of the Board of Longitude, 

 and four of them, with four new sextants, have been ordered 

 by the committee of shipping of the Hon. the East India 

 Company, for trial on board the first four of their ships pro- 

 ceeding to the East Indies. 



Mr. A. has applied a similar telescope and level to the 

 pocket sextant, for the purpose of taking the vertical angles 

 required in surveying, and of thereby dispensing with the use 

 of a theodolite for that purpose. 



Aet. XIX. — On the Optical Illusion of tlie Conversion of 

 Cameos into Intaglios, and of Intaglios into Cameos, with 

 an Account of other Analogous Phenomena. 



The remarkable phenomena to which we propose at present 

 to direct the attention of our readers, while they possess all 

 the interest which belongs to them as physical facts, have at- 



