644 PHILOSOPHICAL TRANSACTIONS. [aNNO 1683. 



eases incident to this organ, with a full account of their causes and remedies. 

 The whole is illustrated by 1 6 plates, in which the several parts of the ear are 

 (for the clearer understanding thereof) represented larger than nature. 



An Account of two Letters of M. Perrault and M. Mariotte, concerning Vision. 

 Printed at Paris, l682. N° 149, p. 265. 



The occasion of these 2 letters was an observation of Mr. Mariotte*s, that 

 any object is not seen when the species light upon the basis of the optic nerve. 

 The experiment on which it is grounded is this : take a piece of white paper 

 of 6 inches diameter, and fasten it on a dark-coloured wall, that it may be 

 level with the eye ; take another small piece of paper, and place it towards the 

 left hand, at 2 feet distance from the former, but about 2 inches higher on the 

 wall ; if you then remove to the distance of 8 or Q feet, and close the left 

 eye, fixing the right on the smaller piece of paper, the larger paper will then 

 quite disappear. 



It is not at all doubted but the image which should appear falls just on the 

 base of the optic nerve; it is also certain that the retina is to be found in that 

 place, but the choroid not ; which gives a very fair suspicion to M. Mariotte, 

 -that the choroid is the seat of vision, and not the retina. 



The novelty of this opinion has found many opposers, and among the rest 

 M. Perrault, whose arguments in the first letters are in short reduced to 3 

 heads. 1 . If the choroid were the seat of vision, its function would be hindered 

 by the branches of blood-vessels lying in the retina. 2. The choroid should 

 not be rugged and unequal, nor hard and thick, nor have a slime or dirtiness 

 on it, to hinder the impression of light, nor want a communication with the 

 optic nerve. 3. If the want of vision in the foregoing experiment may be 

 solved by any of the two probable reasons here offered, then there is no need of 

 discharging the retina. 



To the first of these M. Mariotte answers: that there are defects in vision, 

 caused by the blood-vessels in the retina ; but these defects are not sensible 

 when we look with both eyes, for there are no vessels that lie so near the optic 

 axis as to hinder a direct view, and in an oblique, one eye helps the other, it 

 being difficult for the rays to fall on a like plane in each eye. Again, these 

 vessels that are nearest the optic axis are no thicker than a hair, or the 240th 

 part of an inch, and being in the surface of the retina, are at some distance 

 from the choroid, so as to let rays enough pass under, for the distinguishing of 

 objects not very small. The vessels also that carry the blood are clear and pel- 

 lucid, causing a refraction that is helpful to vision. Here also may enter some 



