VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 557 



equilibrium of the influence, agreeably to what has been already shown, is never 

 disturbed, unless means for its restoration be at the same time afforded. Neither 

 then ought signs of attraction and repulsion to be in this case presented, on the 

 supposition that the influence is electrical ; since it is necessary for the exhibition 

 of such appearances, that bodies, after becoming electrical, should remain so 

 during some sensible portion of time: it being well known, for example, that 

 the passage of the charge of a Leyden phial, from one of its surfaces to the 

 other, does not affect the most delicate electrometer, suspended from a wire or 

 other substance, which forms the communication between them. 



JT//. Observations on the Sl7-uclure of the Eyes of Birds. By Mr. Pierce Smith, 



Student of Physic, p. 263. 



In March, 1792, I observed, while dissecting the eyes of birds, an irregular 

 appearance of the sclerotica, in that part of it which immediately surrounds the 

 cornea, and which in them is generally flat. On a more minute examination, it 

 appeared to be scales lying over each other, and which appeared capable of mo- 

 tion on each other. These appearances I showed to Dr. Fowler of London, and 

 to Mr. Thomson, surgeon, Edinburgh. In June, this paper was copied out at 

 my request, by Mr. Irving, who resided in the same house with me. On inves- 

 tigating this singular structure, the scales were found to be of bony hardness, at 

 least much more so than any other part of the sclerotica. On the inside of the 

 sclerotic coat of the eye there was no appearance of these scales, that part of it 

 being similar to the rest of the sclerotica. Tendinous fibres were detected, 

 spreading over the scales, and terminating at last in forming the 4 recti muscles 

 belonging to the eye; so that on the contraction of these muscles, motion of the 

 scales would be produced. This imbricated appearance, and the detection of the 

 tendinous fibres spreading over scales terminating at last in the 4 recti muscles, 

 led me to consider the use of this structure, what would be the effect of motion 

 of the scales on the vision of birds, and how far this can be applied to other 

 animals. 



It is a fact so well known to persons acquainted with optics, that it is almost 

 unnecessary to mention it, that the rays of light passing through a lens, will be 

 refracted to a point or focus beyond the lens and this focus will be less distant 

 in proportion as the lens approaches a sphere in shape. Now this principle is 

 very naturally applied to the explanation of the use of this apparatus. These 

 scales lying each partly over the next, so as to allow of motion, will on the con- 

 traction of the recti muscles inserted into and covering them, move over each 

 other, and thus the circle of the sclerotica will be diminished, and of course the 

 cornea which is immediately within the circle made by these scales will be pressed 

 forwards, or in other words rendered more convex, and thus the focus of the eye 



