Microscopical Essays, 



57 



extremities of the objea, and unite at the eye in the point F. 

 The angle DFE is formed of the two rays, which at firft proceeded 

 parallel to each other from the extremities of the object, but that 

 were afterwards fo refracted, or bent, by pa'fling through the 

 glafs, as to unite at if s focal point F. C O is equal to the focal 

 diftance of the lens on the fide next the object, C F equal thereto 

 on the fide next the eye, F O the diftance of the eye. 



From the allowed principles of optics, it is evident, that the 

 object would appear double the fize to the eye at C, that it would 

 to the eye when placed at F ; becaufe the diflance F O is double 

 the diftance C O. We have only to prove then, that the angle 

 AC B is equal to the angle IF K, in order to eftablifh the 

 proportion. • 



The optical axis is perpendicular to the glafs and the furface of 

 the object.. The rays A I, B K, which flow from the points A B, 

 are parallel to each other, and perpendicular to the glafs, till 

 they arrive at it ; they are then refracted, and proceed to F 5 

 where they form the triangle IF K, refting qn the bafe I K : now 

 as C F is equal to C O, and I K is equal to A B, the two tri- 

 angles ACB IFK are fimilar, and confequently the angle 

 at C is equal to the angle F. If the vifual rays are continued to 

 the furface of the objea, they will form the triangle DFE, 

 equiangled to the triangle ABC; and therefore, as CO is to A B, 

 fo is FD to DE; and confequently, the apparent diameter of 

 the objea feen through the lens, is double the fize that it is when 

 viewed by the naked eye. No notice is here taken of the double 

 i-efraaion of the rays, as it does not affea the demonftration. 



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