68 LETTERS ON SCIENTIFIC SUBJECTS. 



borne lodge, and the conveniency of a passenger that way, I 

 have advertised these lines to you, desiring to heare from you 

 agayne by the next opportunity you can finde. 



In your tract De Loco Imaginis, &c.,the theoremes you take 

 for principles, undemonstrated, require demonstration, as 

 much as the conclusion you would proove by them, which 

 yourself having first given notice of, I suppose it worth your 

 paines to send their demonstrations : and so to cleere the 

 manner of vision, how it is made, demonstratively ; for as yet 

 we take all upon probability. 



But suppose these theoremes were demonstrated, there 

 is yet one maine doubt remaining ; and that is, how the sense 

 should take notice of the lateral! beames, which only touch 

 on the superficies of the eye, and enter not into it directly, 

 but refracted. And if the sense follow the direction of the 

 refracted beame, that leads it not to the 

 object. As suppose the lateral beame be 

 B O ; the refracted beame in the eye O A ; 

 the doubt is, how the sense can take no- 

 tice of B O without the eye, soe as by that 

 to be descried in the place of the image. 

 Whereas it seemes more probable the eye 

 should be sensible only of the beame O A, 

 which is within it ; but then if it follow 

 the direction of this beame A O, it will 

 lead it to P, against all experience. But it may be, the 

 cleare expression and good proofe of the manner how vision 

 is made, will satisfye this inquisition and cleare the doubt ; 

 I meane the manner how simple vision is made, and how in 

 that the sense judges the object without it to be in such a 

 place, and not short or further on ; for though this forme is a 

 thing evident of itself, yet when I consider it more seriously, 

 I finde it not sufficiently demonstrated by any I have yett 

 read. 



Agayne, supposing the sight to be discovered on the lateral 

 beames, yet it is not thoroughly apparent why it should 

 judge the place of the object to be in the concurse of these 

 beames. You will say perhaps, else it would judge the ob- 

 ject to be in two places. This I well conceive as an absurdity 

 crossing under experience ; but the cause a priori is the thing 

 1 looke for and would have, if it may be had. And indeed 

 this lawe well cleered would necessary ly conclude the former, 

 except single vision may be made on one line, and then the 

 former doubt must be cleered by itself. 



Concerning your other tract of the Prospective. The short 

 time I stayd with you permitted me not to take sufficient in- 



