888 Observations on a new Kind of Spectacles. 
thor, I beg you will refer those who may wish to see the 
original to the Moniteur of the 2 ist of September last. 
I remain, sir, 
Your obliged and obedient servant, 
Nov. 20, 1813. Wn. W OL aa 
Observations by M.Bror on a new Kind of Spectacles in: 
vented by Dr, WouLuastron. 
Every one knows that those whose eyes are too convex 
cannot see distant objects distinctly, because the pencils of 
rays of light intersect each other in the eye before they reach 
the retina. On the contrary, those whose eyes haye too little 
convexity, as 1s generaily the case in old persons, cannot 
see with distinctness those objects that are at a short di- 
stance, because the rays conyerge towards a. point that is 
beyond the retina. The former defect is remedied by the 
use of concave glasses, which remove the focus of rays to 
a greater distance ; ; the latter is relieved by convex glasses, 
which have the effect of shortening the focus. 
But those who have recourse to common spectacles can- 
not see with distinctness, any objects which are not nearly 
in the direction of the axes of the glasses. Objects seen 
remote from the centres are distorted and confused, by reas 
son of the obliquity of the rays to the surfaces of the ‘glass, 
which. occasions a degree of irregular aberration. Hence 
with such glasses the view can embrace but a small number 
of objects at atime.. The head must be moved in sucha 
manner as to direct the axes of the glasses to each object 
in succession, with great inconyenience 1a very many in- 
stances. 
It is now some years, since Dr. Wollaston proposed a 
remedy for this defect by a very simple invention. He re- 
marked that, since the pupil of the eye is of, very small 
size, 1t is m fact but a very small portion of a spectacle 
glass that is employed in any one position of the eye, though 
its several parts are used in succession, when any lateral 
motion is given to the FNC: de thence inferred that the 
form usually given to such Blassese thouy “h well adapted to 
other uses, for which the rays from all parts of the glass 
are to be collected into one focus, is not the best for specta- 
eles; but that the best construction would be that which 
would give to all parts separately the same power of assist- 
ing the ‘sight, when the eye is turned to each of them in 
succession. Dr. Wollaston was thus led to the obvious 
' conclusion 
