594 * Scientific Intelligence. [Mat, 



to be founded upon a false analogy. The eye sees with a distinct- 

 ness wnich agrees with the distance of the ohject : a result which 

 can be determined only by the faculty of vision itself, without any 

 correspondent variety of the focal power. That the focal powers of 

 the eye undergo no change in order to produce vision at ditiereut 

 distances, appears from this fact, namely, that v,'e are enabled to 

 see a great variety of objects at oil distant e^, within a range of 

 perhaps from three yards to three miles at the same time. A 

 thousand objects may be interspersed in this range. Can there, 

 then, exist a correspondent number of distinct focal powers in the 

 eye at the icime time P 



Tue eye requires a determinate focal power-:^ its only movements 

 are for the purpose of regulating the axis of vision : and peicepti( ns 

 are formed according to the relation which subsists between light at 

 various approximaiious to a focus, and the faculty of vision allied 

 with the retina. 



Bath, Jpril 4, 1815. 



V. Proposed Road over HoJtnslow Heath, 



(To Dr. Thomson.) 

 SIR, 

 An Act of Pailiament has passed for the indosure of Hounslovr 

 Heath, and the commissioners have already begun to act upon it. 

 It may not, however, be too late to induce them to lay down one of 

 their roads in the line on which Gen. Roy measured his base. The 

 plan seems unobjectional)le, and it certainly would be attended by 

 circumstances which make the execution of it highly desirable. The 

 suggestion of it in the Annals can at least do no harm, and will 



oblige 



A Constant Reader. 



VI. On Mr. Lockhart's Imaginary Cube Roots. By Dr. Tiarks. 



(To Dr. Thomson.) 

 SIR, 



Having seen in the last number of your Aniials a paper by Mr. 

 Lockhart, which contains this most extraordinary assertion, that 

 every cubic equation has more than two imaginary roots, 1 beg leave 

 to state to you that the imaginary expression which Mr. L. supposes 

 to be a root of 4, different from the two vvell- known roots, is 

 nothing but a different form of one of them. 



Mr. L.'s expression is, ^ K V — (V — a V — 3). 



Now it will be easily seen that — ( V — % V — 3) is the square 



_ /'V.+ .^ ^ ~^\. The above expression, therefore, is, — 



^ 3 -f ^/ - 3 ^ ' ^ V'-) = -- (2 + 2 v/ - 3), which is 



