150 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



order to diverge the rays before they enter the eye, and 

 thus lengthen the focus of the crystalline lens to form an 

 image on the retina. When the eyes are not directed near 

 the centre of the spectacle-glasses, the obliquity o v f their 

 surface to the rays will be increased, so as to occasion a 

 confused appearance of the object. A great portion of this 

 confusion is removed in the spectacles now usually made, 

 when compared with those formerly employed, whose size, 

 being very large, augmented the imperfection ; for it may 

 be observed that when objects are seen through spectacle- 

 glasses, no more of the glass is employed at one view than 

 a portion equal to the size of the pupil of the eye ; this on 

 an average may be reckoned at the eighth of an inch in 

 diameter. Thus, we see how small a portion is used for the 

 purposes of vision ; but as it would be tedious to require 

 the eye always to look through a small aperture, the glasses 

 are left of a sufficient size to admit of a moderate degree of 

 motion ; and as we require a greater latitude horizontally 

 than vertically, their figure is made of an oval, oblong, or 

 octagon form. 



The mountings are usually of gold, silver, and fine steel ; 

 these last rapidly superseding the others owing to their 

 extreme lightness and elasticity, though many are sold 

 which are mere imitations of the genuine article, having 

 little or no elasticity ; the bows are formed to slide, to turn, 

 and with single branches, which last are convenient for 

 ladies not disturbing the hair ; and for gentlemen requiring 

 expedition in placing them on. . Tortoise-shell mounted spec- 

 tacles are also worn for extreme lightness, and by those 

 objecting to metal frames ; are pleasant to wear, but liable 

 to be easily broken. 



In the selection of spectacle-glasses great care should be 

 used in examining them, and the first point of importance is 

 the goodness of the material of which they are formed ; this 

 should be free from all veins or small bubbles, for if one of 

 these occur in the portion through which we look, it will 

 greatly impair the eyes. The next circumstance is that the 

 figure of their surfaces should be perfectly spherical, for if 

 they are curved more in one direction than in another they 

 will injure the sight. The polish should be clean, and free 

 from flare, which too often arises from the manner in which 

 they are usually polished on heterogeneous surfaces, pro- 

 ducing what is technically termed a curdled glass. 



