METHOD OP USING IT. 



supposed to look downwards at o, as if they had proceeded from 

 A' B'. In this manner the observer, looking from E' towards the 

 table, will see an image of the object at A' B', the point A' of the 

 image which corresponds with the top of the object being nearest to 

 him, and the point B', which corresponds with the bottom, being 

 farthest from him ; so that, in effect, the image will appear inverted. 



Now suppose two lines, A' o' and B' o', drawn from the extremities 

 of the image A' B', to a point o' very near to o, and so as to pass 

 through that part of the glass M M' which is not silvered. An eye 

 looking from o' would then see the part of the paper upon which 

 the image A' B' is projected, and would also see a pencil held in the 

 hand of the draughtsman directed to the paper. 



If the distance between the points o and o' be less than the 

 diameter of the pupil of the eye, the observer looking down from 

 E' will see at the same time, and in the same position, the image 

 A' B' and the part of the paper corresponding with it, for he will 

 see the image by the rays which converge to o, and the paper 

 by those which converge to o' ; the "effect, in short, will be that he 

 will see the image as if it were actually projected upon the paper. 



If the eye be advanced towards the mirror, so far as to cause the 

 limiting ray A' o to graze the lower edge of the pupil, the paper 

 will be altogether intercepted by the silvered part of the glass 

 M M', and the observer, though still seeing the image of A B 

 reflected in the glass, will no longer see it on the paper, and for 

 the same reason, he will see neither his hand nor the pencil, and 

 he cannot of course make the drawing. 



If, on the contrary, the eye be moved from the glass so far as to 

 cause the limiting ray A' o to graze the upper edge of the pupil, 

 the image of A B reflected from M M/ will altogether disappear, 

 and nothing but the hand and the pencil will be seen, these last 

 being visible through the imsilvered part of the .glass. 



5. It is evident, therefore, that in order to enable the eye to see 

 the entire image projected on the paper, it must be held in such a 

 position, that while the limiting rays' o', shall pass within the lower 

 edge of the pupil, the limiting ray A' o shall pass within its upper 

 edge. That this may take place, it is necessary that the distance 

 between the points o and o' shall not exceed the diameter of the 

 pupil, and that the eye be steadily held, so that o and o' shall be 

 both within the pupil. 



Since the average diameter of the pupil is two-tenths of an inch, 

 it follows that the distance between the points o and o' should 

 not exceed that limit, and that any displacement of the head 

 which would displace the eye through the space of two-tenths of 

 an inch, would remove from view the pencil or image, partly or 

 wholly. 



187 



