REMARKS O^ SPECTACLES. 195 



foci will, to them, render the extreme parts of objefls fome- Remarks on 

 what confufed; but in a raucli lefs degree than to perfons, with certam thecal uc 

 perfect eyes, or undecayed humours. of Dr. Wolla- 



In telefcopes and microfcopes, the aberration is "dually cut ^"'^^jat^nt 

 off by the infertion of circular apertures or flops; but in fpec- 

 tacles this is not eflenlially neceffary, nor does the want of 

 them or the figure of ihe glafles, prove that they are con- 

 ftitutionally bad and prejudicial. 

 ' The obfervation of Dr. W. that only a portion of the glafs 

 a little larger than the pupil of the eye, is employed at once, 

 is onlyjuft inafmuch as it relates to the mind, being intent on 

 a point of an objed ; but not fo in regard to a general view. 

 For the refra6tive power of the lens does moft admirably colledt 

 all the infinite number of pencils of rays, or cones, into one 

 concurrence at the pupil of the eye ; where they crofs or in- 

 terfe6l each other : yet, fuch is the exquifite fubtilty of light 

 that no confufion or irritation takes place. Man is thus bletfed 

 by affifled vifion, as he is in vitality by the refpiration of air. 



Dr. W*s inferring the form of a menifcus from the (hape of a 

 globe, is manifeftly erroneous ; and in refpe6t to fpe6tacles in« 

 appropriate. A glafs globe or fphere without any fenlible 

 thicknefs, to an eye exa6lly placed in its centre, admits all the 

 incident rays to pafs throughout it unrefracled. If the eye 

 deviates from the centre, a refradion will take place, and that 

 in proportion to the thicknefs of the fphere. Rays of in- 

 cidence pafs perfedly unrefradled through truly ground plane, 

 or parallel glafs, to an eye before k ; and let the axis of the 

 eye, be ever fo much inclofed, unlefs the glafs be very thick, 

 the obje6t will ftill appear perfed, and no refraclion of the 

 incident rays will be obferved. It is obvious, therefore, the 

 nearer a lens approaches to the figure of a plane, the more 

 perfect it mufl: be. 



The figure of a menifcus, which Dr. W. withes to adopt, 

 is as different from a fphere, as a plane. Its figure is com^ 

 pofed of two portions of fpheres, of different radii. When 

 with a pofitive focus it is mathematically demonftrable, that 

 it has entirely the properties of a convex lens, and with a 

 negative focus the properties of a concave one. When the 

 radius of the exterior curve, is lefs than that of the interior 

 it is a convex fort of lens, and magnifies; but, when the 

 radius of the interior curve is lefs than that of the other, it is 



02 a con- 



