PHILOSOPHICAL TRANSACTIONS. 509 



pear in a place not too far ofF nor too near to, but at a convenient distance 

 from the eye ; so that the short-sighted eye shall view the picture of the object 

 in the same manner as if the object itself were in that place. But then be- 

 cause the pictures themselves are inverted, and therefore will be uncouth to 

 one not used to see them in that posture, I considered of these expedients to 

 help that defect also. 



First, if it be only for reading of a book or writing, there needs nothing 

 but the inversion of it, and then holding the convex at a due distance ; for the 

 picture of the letters will appear erected in the due place, for the eye to see and 

 distinguish them very plainly. 



Secondly, for seeing to write, I thought this would be the best expedient ; 

 that the short-sighted person should first learn to read with his naked eye up- 

 side down, which may be quickly attained. 



Thirdly, for distinguishing objects at a distance, I can assert by my own ex- 

 perience, that with a little use of contemplating objects inverted, one shall have 

 as good an idea, and as true a knowledge of all manner of objects, as if they 

 were seen erected in their natural posture. For, in truth, all men have the 

 picture of the object inverted at the bottom of the eye, and not erected, which 

 is the place where they are really seen ; and yet we see by use, that we have an 

 idea of it as if it were erected, and by much use of seeing things inverted, the 

 same idea will be formed as by seeing them erected. 



Of the best Form of horizontal Sails for a Mill, and of the inclined Sails of 

 Ships. By Mr. Hooke. Philos. Collect. N°3, p. 6]. 



The invention of horizontal vanes consists in disposing each of them so, as 

 that they may receive the greatest impression from the wind that is possible in 

 every point of the circumference through which they pass; and the least hin- 

 drance to the motion by the stagnant air behind. This is founded on the same 

 principle with that of the sailing of ships, and other vessels on the sea, viz. 

 on disposing and ordering of the vane or sail so, as to stand in the best position 

 possible to move the arms of the mill or the body of the ship. The first prin - 

 ciple then common to both is, that the vane or sail be as near as possible a per- 

 fect plain and smooth surface, without any bellying or curvity. Secondly, that 

 the air may have as many passages between the parts of the vane or sail as may 

 be, that the moved air may come to it as free as possible without being inter- 

 cepted by a stagnant air before it, to impede or divert its force. Thirdly, that 

 the plain of the vane or sail be put in the middle inclination, between the way 

 of the wind and the way of the arm^ or that of the body of the ship. 



