84) Prof. Encke on Hadlej-'s Sextant. 



probable tliat the shock depends upon some property deve- 

 loped by the action of the nerves. 



To attempt to reason upon any plij3enomena of this kind as 

 dependent upon a specific fluid, would be wholly vain. 



Little as we know of the nature of electrical action, we are 

 still more ignorant of the nature of the functions of the nerves. 

 There seems, however, a gleam of light worth pursuing in the 

 peculiarities of animal electricity, its connection with so large 

 a nervous system, its dependence upon the will of the animal, 

 and the instantaneous nature of its transfer, which may lead 

 when pursued by adequate inquirers to results important for 

 physiology. 



The weak state of my health will, [ fear, prevent me from 

 following this subject with the attention it seems to deserve ; 

 and I communicate these imperfect trials to the Royal Society, 

 in the hope that they may lead to more extensive and profound 

 researches. 



October 24, 1828. Lubiana, Illyria. 



XIII. On Hadley's Sextant. 



{From Prof. Encke's Ephcmerisfor 1830,^.285.) 



TN instruments of reflexion, the measurement of angles is ef- 

 -■- fected by the coincidence of a ray coming directly from an 

 object into the eye, with one from another object thai has under- 

 gone one or two reflexions. The coincidence, in this case, sup- 

 plies the place of the observation of the second object in other 

 instruments for measuring angles by direct vision, and does 

 away the necessity of investigating, whether the radius detei- 

 mined by the first reading has remained unchanged, or at least 

 parallel to its former position. In the case of reflecting in- 

 struments, we have only to regard the angles which the dif- 

 ferent lines form with each other, and not their absolute posi- 

 tion in space; and it will therefore be sufficient, instead of the 

 real lines, to introduce lines parallel to them all, passing through 

 •one point ; and in this manner all the investigations relating to 

 them will be converted into pioblems of spheriqal trigono- 

 metry. 



Let O (fig- 1.) be the centre of the division, and let a sphere 

 of any diameter be described about this point. The intersec- 

 tion of the divided plane of the sextant with this sphere will 

 be a great circle. If the divided arc is to show the correct 

 value of every angle, the direct and tlie reflected rays, the lat- 

 ter of which supplies the place of the second bisection, must 

 be entirely in this plane, from which we derive these conditions 



of 



