VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 411 



broad and flat, arising from tiie back part of the orbit, passing over the ball as 

 over a pulley, and inserted by broad flat tendons at the anterior part of the eye ; 

 the oblique inserted toward the posterior part, also by broad flat tendons ; when 

 they act jointly, the eye being in its horizontal position, it is obvious, as every 

 muscle in action contracts itself, the 4 recti by their combination must neces- 

 sarily make a compression on the different parts of the eye, and thus elongate its 

 axis, while the oblique muscles serve to keep the eye in its proper direction and 

 situation. For my own part, I have no more difiiculty in conceiving of this 

 combination of those muscles, than I have at present of the different flexors of 

 my fingers in holding my pen. But other corresponding effects are also pro- 

 duced by this action ; not only the distance between the anterior and posterior 

 parts of the eye is increased, but of consequence the convexity of the cornea, 

 from its great elasticity, is also increased, and that in proportion to the degree of 

 pressure by which the rays of light, passing through it, are thence necessarily 

 more converged. But another effect, and one not inconsiderable, is, that by 

 this elongation of the eye, the media, viz. the aqueous, crystalline, and vitreous 

 humours through which the rays pass, are also lengthened, of consequence their 

 powers of refraction are proportionably increased ; all which correspond with the 

 general principle. It may however be said, that as the 4 recti muscles are larger 

 and stronger than the 2 oblique, the action of the former would overcome that 

 of the latter, and thus draw back the whole globe of the eye ; but does not the 

 fat at the posterior part of the orbit also afford a resistance to the too great ac- 

 tion of the recti muscles, especially as it is of a firm consistence, and the eye 

 rests immediately on it ? Admitting then that this is the operation of the exter- 

 nal muscles when in a state of contraction, it is also to be observed that we 

 have the same power of relaxing them, in proportion to the greater distance of 

 the object, till we arrive at the utmost extent of indolent vision. 



But, as a further testimony of what has been advanced, I had recourse to the 

 following experiment, which will show that the eye is easily compressible, and 

 that the effects produced correspond with the principles I have endeavoured to 

 illustrate. With the common speculum oculi I made a very moderate degree of 

 pressure on my eye, while directing my attention to an object at the distance of 

 about 20 yards ; I saw it distinctly, as also the different intermediate objects ; 

 but endeavouring to look beyond it, every thing appeared confused. I then in- 

 creased the pressure considerably, in consequence of which I was enabled to see 

 objects distinctly at a much nearer than the natural focal distance ; for example, 

 I held before my eye, at the distance of about 2 inches, a printed book ; in the 

 natural state of the eye I could neither distinguish the lines nor letters ; but on 

 making pressure with the speculum I was enabled to distinguish both lines and 

 letters of the book with ease. 



3 G 2 



