OK THE MECHANISM OF THE EYE. 



581 



radiating direction. For this purpose, the 

 best object is a candle or a small speculum, 

 viewed through a minute lens at some little 

 distance, or seen by reflection in a larger 

 lens. If any pressure has been applied to the 

 e^'e, such as that of the finger keeping it 

 shut, the sight is often confused for a short 

 time after the removal of the finger, and the 

 image is in this case spotty or curdled. The 

 radiating lines are probably occasioned by 

 some slight inequalities in the surface of the 

 kns, which is very superficially furrowed in 

 the direction of its fibres: the curdled. ap- 

 pearance will be explained hereafter. When 

 the point is further removed, the image be- 

 comes evidently oval, the vertical diameter 

 being longest, and the lines a little more dis- 

 tinct than before, the light being strongest 

 in the neighbourhood of the centre ; but im- 

 mediately at the centre there is a darker spot, 

 -owing to such a slight depression at the ver- 

 tex as is often observable in examining the 

 lens after death. The situation of the rays 

 is constant, though not regular; the most 

 conspicuous are seven or eight in number ; 

 sometimes about twenty fainter ones may be 

 counted. Removing the point a little fur- 

 ther, the image becomes a short vertical 

 line ; the rays that diverged horizontally be- 

 ing perfectly collected, while the vertical 

 rays are still separate. In the next stage, 

 which is the most perfect focus, the line 

 spreads in the middle, and approaches nearly 

 to a square, with projecting angles, but ip 

 marked with some darker lines towards the 

 diagonals. The square then flattens into a 

 rhombus, and the rhombus into a horizontal 

 , line unequall}' bright. At every greater dis- 

 tance, the line lengthens, and acquires also 

 breadth, by radiations shooting out from it, 

 but does not become a uniform surface, the 



central part remaining always considerablj 

 brightest, in consequence of the same flat- 

 tening of the vertex which before made it 

 faintest. Some of these figures bear a consi- 

 derable analogy to the images derived from 

 the refraction of oblique rays, and still more 

 strongly resemble a combination of two of 

 them in opposite directions ; so as to leave - 

 no doubt, but that both surfaces of the lens 

 are oblique to the visual axis, and cooperate 

 in distorting the focal point. This may also 

 be verified, by observing the image delineated 

 by a common glass lens, when inclined to the 

 incident rays. (Plate 12. Fig. 92. n. 28. .40.) 

 The visual axis being fixed in any direc- 

 tion, I can at the same time see a luminous 

 object placed laterally at a considerable dis- 

 tance from it; but in various directions the 

 angle is very different. Upwards it extends 

 to 50 degrees, inwards to 60, downwards to 

 70, and outwards to QO degrees. These in- 

 ternal limits of the field of view nearly cor- 

 respond wjjh the external limits formed by 

 the different parts of the face, when the eye 

 is directed forwards and somewhat down- 

 wards, which is its most natural position ; 

 jflthough the internal limits are a little more 

 extensive than the external : and both .are 

 well calculated for enabling us to perceive, 

 the most readily, such objects as are the most 

 likely to concern us. Dr. VVollaston's eye has 

 a larger field of view, both vertically and ho- 

 rizontally, but nearly in the same propor- 

 tions, except that it extends further upwards. 

 It is well known, that the retina advances 

 further forwards towards the internal angle 

 of the eye, than towards the external angle ; 

 bi^t upwards and downwards its extent is 

 nearly equal, and is indeed e^very way greater 

 than the limits of the field of view, even if 

 allowance is made for the refraction of the 



