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point of an object, penetrate the chrystalline humour, 

 that so an image of that object may be painted there. 

 In order to this, the rays striking on the cornea, are 

 reflected toward the perpendicular, and thus directed, 

 through the pupil to the chrystalline. Mean time 

 the iris, contracting or dilating the pupil, admits 

 fewer or more rays, as the object is more or less vivid. 



Now the flatter the cornea is, the fewer rays does 

 it collect and transmit to the chrystalline, and those 

 more diverging. The rounder it is, the more rays 

 does it collect and transmit, and those more converg- 

 ing.* It is too flat in old men; it is too round in them 

 that are short-sighted. The rays transmitted through 

 the pupil to the chrystalline humour, are th^re re- 

 fracted anew, collected and rendered converging, and 

 those that come from the same point, are thrown in 

 one point on the bottom of the cyo. But if the chrys- 

 talline be too dense, the focus (or point wherein they 

 unite) will be too near : if that bs not dense enough, 

 it will be too remote. And this is another cause of 

 short-sightedness or the contrary defect. 



In all vision both the eyes are used at once. And 

 both together (as any one will lind upon trial) be- 

 hold an object in another situation than either of 

 them apart would do. Hence a gentleman who had 

 one of his eyes struck out, for some months after 

 was apt to mistake the situation of things, and when 

 he attempted to pour liquors into phials, often poured 

 them quite beside the neck of the phials. 



Two eyes greatly contribute, if not to distinct, at 

 least to extensive vision. When an object is placed 

 at a moderate distance, by tiie means of both eyes 

 we see a larger share of it than we possibly could 

 with one ; the right eye seeing a greater portion of 

 i.ts right side, and thj left eye of its correspondent 

 tide. Thus both eyes in some measure sec round the 

 object ; and it is this that gives it in nature, that 

 bok relievo, or swehsng, with which it appears ; and 

 which no painting, how exquisite soevoi, can attain, 

 to. The painter must be contented with shading on 



