424 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 7 ] Q. 



being 2 miles south of Holderness, in the River Humber, and 4 miles north of 

 Lincolnshire, &c. 



^ Way for Myopes to use Telescopes without Eye-glasses, By the Rev. J. T. 

 Desaguliers, LL.D. and F. R. S. N° 36l, p. 1017. 



Myopes may use telescopes without eye-glasses; an object-glass alone being 

 as useful to them as a combination of glasses, and sometimes more so. 

 Lemma I . — What is required of a telescope is to give large and distinct vision, 

 that is, to make the object, as in Galilaeo's telescope, or its image, as in the 

 telescopes made up of convex lens, appear under a great angle, and to have all 

 the rays of those pencils that enter the eye, meet in a point on the retina of 

 the eye, on their respective axes. 



Fig. 2, pi. xi, represents the combination of two convex lens for the astrono- 

 mical or inverting telescopes; where the above-mentioned requisites are 

 obtained,^ ab is the object, supposed at a vast distance from the objective lens 

 LL, so that rays coming from the extremity a of the object, will fall on the lens 

 LL, in the same manner as if they were parallel to their axis ax, and after 

 passing the glass unite at a, where they project the image of the point a; from 

 whence diverging, they fall on the eye-glass 11, and having passed through it, 

 go on parallel to each other, and enter the cornea of a common eye e, which 

 unites those parallel rays on its retina err at a, where the image of a is pro- 

 jected. The same may be said of the rays that come from b, and after their 

 several refractions through the two glasses, and the coats and humours of the 

 eye, meet on the retina at (3, where they project the distinct image of the point 

 b. The rays that come from all the points of the object ab, being affected 

 after the same manner, give a distinct image of those points on the retina, and 

 therefore the object appears distinct. 



The object will also appear magnified in the same proportion as the angle 

 Icl = to bMa, under which its image is seen, is greater than the angle acb, 

 under which the object ab would be seen by the naked eye; as is more at large 

 demonstrated by dioptrical writers. 



Lemma 2. — If parallel rays fall on the cornea of a myopes, or short-sighted 

 person, they will unite in the eye before they come to the retina, and the far- 

 ther from it the more convex the eye is; but if the rays which fall on the cor- 

 nea diverge in proportion to the too great convexity of the eye, as from d, 

 such rays will be so refracted by the coats and humours of the eye, as to meet 

 in one point on the retina rr, see fig. 4 and 5. Where I have neglected the re- 

 fraction of the rays passing out of the crystalline k into the vitreous humour v, 

 as I do in the other cases. 



