Account of a new Optometer. 11 



opportunity of examining the eyes in different classes of animals, 

 and have come to the conclusion that the retina does not cor- 

 respond in form to a caustic surface of revolution, but that its 

 form is in a greater or less degree determined by the amount of 

 mobility required and by the construction of the head, or, in 

 other words, simple adaptation to what is suitable to the habits 

 and instincts of the animal. I have likewise in the most careful 

 manner separated the lens under water from its sclerotic attach- 

 ments, suspended it in the water, shut off extrinsic light, and 

 searched for the focus of parallel rays with different sizes and 

 different arrangements of diaphragm, but have totally failed to 

 discover it : nothing has been noticed but an ill-defined diffused 

 brightness, rather more brilliant in the middle ; nothing to force 

 upon the mind the conviction that it acts as a common corrected 

 lens would do ; but as the experiments, however carefully made, 

 carry with them a suspicion of inexactness, I do not venture to 

 affirm this too positively. 



The eye, like other organs of sense, has its active and passive 

 states ; it is often no more sensible of the presence of external 

 objects than the ear to accustomed sounds, or the skin to the pre- 

 sence of the clothes ; but the moment attention is excited it is 

 roused into full activity, and perception becomes possible ; the 

 iris, controlling the size of the pencils, the lens concentrating 

 the impulses, then determine the exact conditions favourable to 

 vision, and, it may be said, act such a part as an ear-trumpet 

 to a dull ear, by supplying to the retina the exact impressions 

 that are required to be conveyed to the sensorium. 



A priori, it ought not to be expected that the retina would 

 present a simple surface on which to receive a picture; the 

 waves which compose white light are of different lengths and 

 arrive with different velocities, so that, leaving the source of 

 emanation at the same instant, they must arrive at the retina in 

 very different phases of their length, requiring a certain depth 

 of retinal matter for their full development, a depth not less than 

 1^ the length of a medium wave, or about 0*00084 millim. 

 This is just one-fiftieth part of the depth of the rods and bulbs, 

 which are with good reason believed to be the sentient receptacle 

 of the retina. There is therefore ample depth in the body of 

 the retina for the reception of rays from whatever group of ex- 

 ternal objects the mind be prepared to select. It would seem 

 probable, therefore, that the impressions are received within the 

 substance of the retina, and that the iris and lens between them 

 regulate the intensity of the impulses : time moreover is cer- 

 tainly required by the retina for the impulses to become adjusted 

 to meet the requirements of vision, part of this time being 

 known as " personal equation," the whole of it being no more 



