426 Mr. It. Templeton on the received value of p, 



further distance of more or less distinct vision, gradually dissol- 

 ving into a diffused glare. 



4. With larger holes it is not so easy to fix on the exact point ; 

 but it may be remarked that with all eyes there is a point where 

 exact definition ceases ; it is usually at about one and a half to 

 two and a half metres from the eye. 



5. The mean of the above logarithms is 7*7454, correspond- 

 ing to an angle of 19' 8"; and it may reasonably be assumed 

 that in a constant light the hole invariably subtends, at the dot 

 or other object looked at, such angle when the dot or object is 

 perfectly well seen. 



6. If the pupil be dilated with belladonna, extraneous light 

 carefully excluded, the other eye closed, and the experiment 

 again tried, vision is found to be perfectly restored with any one 

 hole at the distance corresponding to that hole. 



7. When the edge of a little fragment of silvered glass is 

 brought up to the eye (that is, held at about 10 to 15 centime- 

 tres from it), so that the pupil can be examined while the eye runs 

 along a vista of objects,, it will be found that the eye can be 

 brought to bear on an object at any particular. distance without 

 distinct vision occurring, unless the act be accompanied by voli- 

 tion ; but the moment the mind wills to see the object, the iris 

 obeys the impulse, contracts or expands as the case may be, and 

 the object is clearly seen. There is a noticeable interval between 

 the exercise of the will and the movement of the iris c The iris 

 would seem to exercise no influence on vision further than me- 

 chanically controlling the brightness of the spectrum on the 

 retina. 



8. If a finger be held up between the eye and the landscape 

 in the background and steadily looked at with one eye, the land- 

 scape is not lost sight of, though not definitely seen; parallel 

 rays, therefore, as well as those diverging from the finger, are 

 impressed upon the retina at the same moment ; yet the difference 

 of foci under these circumstances is fully a millimetre. The 

 depth of the rods and bulbs (the sentient portion of the retina) 

 is, however, only '044 millim. or thereabouts. 



9. It has been shown that, within ordinary reading- distance, in 

 a good light, from any point of an object a conical pencil of rays, 

 the vertical angle of the cone being 19' 8"*, falls upon the 

 cornea and is transmitted through the eye, undergoing therein 

 refractions which bring the rays of which the pencil is composed 

 to a focus behind the lens. If the focus for parallel rays be 

 imagined to be at the retina, it is utterly impossible that the 

 focus for these rays of a conical pencil can ever be so near as the 



* That is, more or less, varying with the individual and the light. 



